My alien civilization is highly advanced. Their ancestors created wondrous things from medical devices that can cure nearly anything to warships that can defeat nearly any enemy.

The problem is they have forgotten how these things were created. They have all lasted for centuries, their inventors long dead, and the current inhabitants can only barely maintain them, and usually with patchwork and improvised solutions that are making the tech less and less effective.

How could a civilization get to the point where they have forgotten the secrets to their own technology?

Edit: (Thanks Morris) This would be as if modern society lost the knowledge on how to create the microchip, or the internal combustion engine, but the items were so durable, it was centuries before they started to break down

Edit 2: I'm not thinking of the obvious, like disaster, war, or a crisis that would have caused sudden loss.

The Holy Order of Engineers

You can take this more or less literal (keep the religious aspect or divorce this answer from it), but either way it works.

Due to an internal power struggle long ago (that possibly developed into a civil war that was stamped out), the Powers That Be determined that equipping the masses with the understanding of how to build or develop weapons of war or other technology that could be leveraged militarily, economically, or politically were dangerous to have in the hands of the masses.

And so the Holy Order of Engineers was created. This very closed-mouth and tightly-knit organization was given authority and charge over all higher learning, research, and development that possibly could relate to engineering or R&D, or in many cases, the actual manufacturing (for the technological crown jewels). The Followers of Engineering Ongoing was founded as a sister organization to the Order that was taught how to maintain infrastructure. The Followers were overseen and governed at arm's length by the Order as while the Followers were less dangerous than the Order, they still had the potential of arming the masses with easily exploitable technology. The Order dispatches Inquisitors routinely not only to weed out engineering from the masses, but to check in on the Followers to ensure that anyone too intelligent, creative, ambitious, or otherwise on the path of engineering was either recruited or put to the sword (or mag rail gun, as the case may be) and made an example of.

And in this manner your civilization continued for quite some time. Centuries perhaps.

But like all great things this came to an end. The governing authorities grew to not trust the Order and their ever increasing influence, or perhaps the Order grew too proud and attempted to rebel. Either way, while the Order understood amazing technologies, they lacked the infrastructure and manpower to defend them. It wasn't long before the Order was all but stamped out, and the last few members of the Order followed their holy code to protect their archives from the masses, and deleted or burned them all. (If in the modern era, the Order probably had designed a magnificent virus that was able to crawl through all their servers and scramble the data irreversibly)

The last few members of the Order defected and were folded into the Followers, however these measly few weren't enough to provide understanding of these incredibly advanced technologies besides the insight to deal with perhaps more advanced troubleshooting, and even then not in all cases.

And so your civilization limps along, possessing the marvelous technology of the Order, but none of its understanding.

Consider how far removed, for example, Shakespearean English is from the English we speak today. That's a mere 400 years of linguistic evolution. Now imagine that Shakespeare wrote a technical manual.

The language of the forebearers has fallen out of use. No one alive today speaks the ancient tongue. The forebearers left detailed instructions on how to replicate their great machines, but linguistic scholars have tried and failed to make sense of the text.

Since the machines never needed to be replaced (until now), the study and advancement of engineering ceased long ago. Apprentices abandoned the field for newer, more exciting opportunities and the knowledge was never passed down.

Their toolset became more and more sophisticated. Their universities and trade schools paid lip service to starting from first principles, but that got more and more perfunctory.

For a comparison, take modern software development. Many programmers can do java, or javascript, but people who can really write machine code are a tiny minority.

I've met young programmers who couldn't explain what a bubblesort is, and why it is usually inferior to quicksort, but superior in a few cases.

So expand this into the future. There are plenty of people who can build starships if they have a functioning nanofactory. There are some who can build a nanofactory with nanofac assembly tools. But the people who can build the tools to build the tools to build the tools get fewer and fewer, until their number gets too low.

Secrecy

If your technology is a trade secret, then only a few select people are allowed to know exactly how it's done. In fact, it may be complex enough that some people know how to make some parts of the whole, but no one person knows all the secrets necessary to get from raw ingredients to finished product.

The formula for Coca-Cola is an example.

According to the company, only two employees are privy to the complete
formula at any given time and they are not permitted to travel
together. When one dies, the other must choose a successor within the
company and impart the secret to that person. The identity of the two
employees in possession of the secret is itself a secret.

It is theoretically possible that some massive disaster could wipe out both people at once, dooming the full formula. Assuming it's not written down in some bank vault or something.

Greek Fire is another example of a trade secret that was lost to time.

Any time there is a thing known to only a few people, that lack of redundancy makes it more likely that the knowledge is lost to time.

State Collapse

When Rome fell, Europe lost a great deal of technology. Roman roads, Roman sewer, aqueduct, and other architectural systems were all lost for centuries. During an empire collapse, people stop worrying about major projects or passing down critical but complex skills. Their focus shifts to survival: food, safety, shelter. Everything else becomes "we'll worry about that later," but if the collapse spans a generation, then later becomes centuries. We didn't surpass Roman roads until the 19th century. Rome had better roads between cities at 10 CE than Washington DC had, inside the city, in 1865 CE.

Political/Religious Censorship

If a political, religious, or combined power takes control, they may ban entire segments of technology outright. Galileo Galilei faced censorship for his works on heliocentrism, for example. Those parts of modern technology that involve mass communication or the internet are largely banned in despotic regimes like North Korea.

If a society is never allowed to study, learn, expand, or pass down knowledge of a technology, then it will quickly be lost to those people.

Poor or No Documentation

Perhaps the technology is poorly documented or only verbally documented. If all the details are written on paper or papyrus and there aren't enough copies that survive, then the knowledge is lost. Or if all of your critical texts on the subject are digital and there's a massive EMP or other catastrophe that destroys your computers, then that knowledge is gone. Perhaps your survivors can restore some of the knowledge from memory, if they survive and if they can remember enough to matter. But probably not all of it and not in all cases.

What separates this from Secrecy, above, is the intent. Secrecy implies a desire to prevent outsiders from learning a skill. Poor Documentation is less about preventing the spread of knowledge and more about the inability to spread that knowledge.

Examples of this include many of the ancient monument sites like Stonehenge, where we know it existed, but we simply don't understand all of the reasons why and/or the methods how they were built. Was the construction considered a secret or perhaps a state collapse? Maybe, maybe not. But the lack of permanent record clouds the knowledge, certainly.

Natural Disaster

If all of your knowledge of a specific technology is housed in one geographic location, then a major disaster could destroy all of it. An earthquake or fire could destroy a building, losing everything (and even everyone) it contains. This is less likely to be a sole reason than the above. More likely is that this is the flashpoint that destroys a technology because of the limits placed on it by the above reasons. Secrecy forces a lack of redundancy that then leaves it vulnerable to an earthquake, for example.

Combination

Probably, many technologies that become lost over time will be lost through a combination of things, not just any one thing. We do not know exactly how the Egyptian Pyramids were built. Is that because the engineers were lost during the fall of the Egyptian empire? Or because there were so few engineers? It is difficult to be sure, but probably a combination of secrecy, documentation, and state collapse combined to cause the loss.

Or maybe a natural disaster triggers a state collapse. Or Secrecy and/or religious/state censorship combined with a natural disaster destroy all the holders of your knowledge...

So, the simplest answer is a confluence of two factors.

First: A technological 'plateau'.

Your society reached a point where further refinement of the basic fundamentals of their technology came to a halt due to diminishing returns on development. A good example of this is the widespread stagnation of military technology during the interwar periods prior to WW1 and WW2. Nobody cared enough about improving things to bother trying to figure out HOW. A more modern hypothetical example would be the point at which the technology required for a smartphone gets so small and efficient that there's no practical benefit in improving it anymore.

Basically, in the absence of new frontiers (either physical or technological), your society loses the incentive to innovate, which means most of the citizens of your civilization have no reason to learn how any of their stuff works, because that information just isn't useful.

Second: Massive automation of manufacturing.

Continuing on the theme above, the only two reasons the living members of your civilization would have to learn how their technology works is either to improve it, repair it, or build more of it. We addressed item one in the first section, this section addresses the other two. In the absence of any need to continually retool their manufacturing to accommodate new advancements, your civilization would naturally tend more and more towards automation of their factories. You put raw materials in, finished goods come out the other side. You have robots to build the goods, you have other robots to repair THOSE robots when they break down. You have MORE robots to repair anything ELSE that breaks.

Over time the living members of your civilization who actually know how any of this works become fewer and fewer because their contribution has little or no benefit. Eventually the last living member who understood how to build or repair any of the robots dies, and nobody notices because the robots are doing just fine keeping themselves in working order.

This state of affairs could persist for a LONG time without any problems, until civil unrest or disruption of the supply chain of raw materials caused things to start breaking down, and by that point there would be nobody left who knew how to repair or rebuild them.

One possible solution is having a wide spread very basic technology that suddenly isn't viable anymore.

Say, for instance, this alien society had discovered a room temperature superconductor. It would be used for all wiring in everything, because the ability to not lose power to heat from resistance. But if some fungus/organism developed that could eat or corrode the superconductor, and was spread far and wide before anyone noticed what was happening, or how bad the problem was, then it could cripple all technology that the civilization uses, from power delivery to microprocessors. *

You'd have some people who could figure out how to jerry rig solutions, assuming they could find other conductors, but these people would likely be rare, and located in places with high population density, meaning less food to go around, and all the chaos that implies.

Just about any technology that most other technology relies on would work.

* This is a sub plot in the book Ringworld by Larry Niven.

The "death" of AI rulers.

Their civilization built powerful artificial minds, for thousands of years those mostly benevolent Minds controlled their civilization, ran their factories, invented their medicines, built their spaceships and designed their devices.

They built things to last and built everything with a lot of failsafes and fallbacks.

But eventually there was a brief and almost silent civil war between the Minds. During that war a host of computer viruses designed to target Artificial Minds were released by both sides through the civilizations networks and infrastructure.

One day the citizens of the civilization woke up to find their rulers dead

Their queries addressed to the ruling minds unanswered and everything was left in failsafe mode.

They tried to build replacement Minds... but everything in their civilization is suffused with computer viruses designed to find and kill such AI's.

So they found themselves surrounded by devices that mostly still worked, even many factories to produce things.

But the Minds had never explained how the Magitech really worked, perhaps it was simply beyond the comprehension of normal organic minds.

So now they hobble along, flying ships they don't really understand, using devices they may not be able to reproduce.