Go for the simplest solution: millennium bug.

You would think that humanity would have learned after Y2K. Nope. In another post I described how GPS has a millennium bug every 19 years or so. This is due to a design flaw, and the fix is people having to buy new GPS devices every 19 years. We are now in the first generation of devices which may survive this because cell phone apps and OS's are patchable for this, but a lot of standalone GPS devices did go nuts on April 6, 2019 and are now useless. The next time this bug will hit will be around 2038.

So it may be that your biotechnology devices have an embedded millennium bug in the firmware which everyone forgot about because no one cared. Maybe fixing it would be too expensive, requiring everyone to change their parts. Every corporation was waiting for someone else to invest the time and money to fix it. This went on for long enough that the risk eventually faded from memory.

Now no one knows how to access firmware, and thus it can't interface with flesh anymore.

By the way, a simple fix would be to reset the date in the devices. Unfortunately no one knows how to do that.

• Change of diet: the implants need some chemicals/proteins/etc. to work, or to be able to interface with your biology. Something wiped out a kind of a crop, that forms the major part of your societys diet, that was the only source of this protein. Or the opposite, some company introduced a new soda, that became such a big hype, that almost everybody tried it. It took some months/years for a compound of this soda to render the interface between implant/biology non-functional.


• EMC: there was a major electromagnetic event (solar storm, terrorist EMP, name your thing), that went unnoticed by your brightest or swept under the carpet by the government. (Or a radiation event, like a few weeks ago in Russia.) Your implants do not have electromagnetic shielding, since it would make them look worse/reduce their usefullness, they were more sensitive to this event than your normal devices. Or this event was frequency selective and only your implants were sensitive to it.

As I see it, for the effect to be quite sudden and global, there are two options

1) outer influence, new kind of radiation, aliens, gods, magic appearing out of nowhere you name it. Generally not very plausible, but there are stories set in worlds in which such a game-changing event occurred. If the setting is interesting enough, the question how it came to existence can be handwaved to an extent.

2) something contagious

2a) some disease/virus. Not very plausible, as generally any change in brain chemistry/structure so strong that it affects implants in an irreparable way would probably be conflicting with normal brain function way too much.

2b) some kind of mass hysteria.

I am imagining this scenario:
A new implant was created not long ago, maybe something directly influencing mood. Basically everyone got one, because it was cheap and sooo handy. Unfortunately it was not tested thoroughly enough. After some years of use it causes not only irreversible changes in brain chemistry (that might be easy to cope with, you simply adjust old implants to this change), but unpredictable fluctuations, peaks, rapid changes thereof, so basically any adjustment you make to any of your implants will let it work for an hour or half a day before your brain fluctuates out of sync. When this happened to the first affected person and the news spread, other people soon became scared (back to mass hysteria point), which significantly strenghtened the effect up and sped up its onset. Eventually this panic caused even those who haven't got the original faulty implant to become affected. Check out wikipedia on mass hysteria and imagined illness.

Not sure if it makes much more sense than alien force field, though.

The biologicals are not actually still biologicals.

The entirety of human civilization was copied and continues to run as a Matrix-like simulation. The event 5 years ago was actually the first boot of the copy. Failure of the implants was intentional and meant to distract from other imperfections in the copy.

The fate of the biological originals is not clear - maybe they perist and do not know about the copy, or maybe they were wiped out, or maybe (again like Matrix) they are being put to another use.

Who cares?

No-one reading your story actually cares what the reason is. What we care about is the story you tell about the consequences. The reason is a MacGuffin which doesn't have to exist in any defined way.

Think of the classics. You don't need to know how Victor Frankenstein connected up the nerves and blood vessels, only that he did, and this is how his creation behaves. You don't have know how the Dune Navigators figure out piloting, only that they do. You don't really care how the Snow Crash Burbclave residents make money and how the Burbclaves negotiate, only that they do, and this is how their society works. Or even closer to your scenario, the post-human Machine starts to break down, and the post-humans have to work out what to do when The Machine stops, but no-one needs to read about the detail of the Machine's repair procedures.

The tech stopped working. Post-human has to work out how to become human again. That's a brilliant hook for a story I want to read when you're done. Why it happened is unimportant to the story - give me the most basic Scotty/LaForge handwavey bullshit if you really want, and move right along.

The radio spectrum is full

The implants are just the peripheral that links the biological part with the actual user device (smartphone, smartwatch... whatever they use in the future). That devices are where the processing power really is, the implant simply serves as a link with it, over low-frequency radio (think Bluetooth).

They worked very well initially, when just a few people had implants, and they hold at most 1-2 implant-linkable devices. But then, we got to a situation where everyone had an implant operating on that frequency. Moreover, due to customer demand everything you buy now is implant-capable: your TV, doors, windows, fridge, bulbs... all of them are smart-things that you can control from your implant.

Five years ago the predominance of these items got to its peak, leading to the spectrum being saturated, and implants no longer working (or, more exactly, they work errantly. If you are lucky it might work, but more often than not, it will be unable to communicate with it).

They are all working on a free range of the spectrum, similar on how your garage opener works at the same frequency as your WiFi and your microwave nowadays and, while it doesn't reach too far, it is a very limited range where they can operate, mainly due to the low power requirements. Even if you were willing to change your implant in order to use a different frequency, going to an higher power one would likely fry your brain (in fact, some of the implants that were manufactured in the later years, were using more energy than advisable in order to produce a working signal -and not just as a punctual event, but continuously-, and are quite risky to use). These people were cyberpunks, not idiots.

Theoretically, you could get them working if you were able to restrict the amount of implant-devices in your surroundings (it is rumored that extremley rich people is able to block external signals and have a strict control of devices operating on those frequency -including the implants of their own employees-). However, that's not something that is achievable by an individual, as that spectrum would be used anyway by your neightbours' devices. You are facing a tragedy of the commons that killed the implant era.

The bio-cyber implants can only interface with someone infected with the otherwise generally asymptomatic Handwaveosis, which makes the brain's neurons particularly sensitive to Macguffin fields. And early bio-cyber implants would only work for a few hours before the body rejected the foreign object - only implants coated with Handwaceae spores avoid forming scabs around them.

So when that awful disease / cruel bioweapon swept through the population and wiped out that vital link in the chain, people's bio-cyber implants stopped working.

By putting the fault in the thing that doesn't exist in real life, you don't have to explain everyone's brains changing - or a society being smart enough to develop the electronics and software not being able to maintain it when it breaks.

You could also blur out the cracks in this story: Perhaps the cause of the problems hasn't been discovered yet, or it's a government/trade secret. So your protagonist has only heard a range of contradictory rumours.

Failed to renew the domain name

It's just one of the old-fashioned things again. All the implants are cloud-connected, of course. That's how they talk to each other, and smooth the communication between their human hosts. And therefore, they all have the same hostname programmed into them. It would be a real modern Babel otherwise, if your implant could only communicate with implants from the same manufacturer.

And on the fateful day, the DNS registration lapsed. That's the sort of accident that's been happening to the biggest of companies. Microsoft forgot to renew Hotmail back in 01999, and that wasn't exactly the last company to drop the ball that way. And that was the easy case, with a domain that belonged to a single company. But these implant manufacturers had teamed up and registered the domain name together.

You know what happens when everybody is responsible? Then nobody is. Sure, each company promised to renew the name in turn, for 3 years. And after 3 years the next company would pay up. The one thing they overlooked in the arrangement (and it's easy to say this in hindsight) was how company mergers would work. Oops. And since this whole arrangement worked for decades, it's not like any of the original decision makers were still around. It's just a lowly manager cutting duplicate expenses after a merger.

So, no hostname, no cloud access, and all implants go in panic mode. They're nice enough not to kill their hosts, but they can't even get the medical dossiers of their hosts, let alone communicate with other implants. Better shut down and do no damage, then. The original programmers were that smart. They just could not foresee how utterly dependent future humans would be on these implants.

In todays world the main barrier to direct brain-machine interfaces is that the body rejects implants. Typically the body will grow a barrier/capsule around any foreign object placed into the brain (in order to isolate it). Presumably they have solved this problem in your world.

That all changed when the I386 virus came into contact with the first human.
The presence of the virus slowly makes the human nervous system more sensitive to foreign objects. The high-tech bio-compatible implant materials are no longer seen as compatible by our bodies. The body then does what it normally does and isolates the implants with a layer of tissue, rendering them useless.

Sure the implants can still collect body heat and sugar from our cerebral fluid to keep operating, but without neural connections they can't do anything.

The virus only existed in 1% of the population that was living isolated from normal society. Past generations never knew about the virus it because it has no noticeable effect on normal humans. The worst part is that the virus is contagious for a long time before it causes any symptoms. By the time it was discovered millions had been infected.

The virus is an NRA type virus and mutates rapidly. By the time anyone found out what was happening there were already hundreds of variants and it was impossible to vaccinate.

Although the question is open ended the answer is far more limited. No matter what, it has to be the biological connection to the mechanical part that prevents a good interface. The mechanical part can be adapted to interpret the nerves again, but if the nerves can interpret the mechanical part the system fails. That limits the options to anything that changes the neurotransmitters or the nerve itself.

A virus that causes neurological damage is out of the question, it would not just kill the interface but also the connection between your own nerves and kill you or severely handicap you in the process. This also means that anything affecting the neurotransmitters is out of the question too.

That leaves the nerves themselves. And the only option they have is to either grow or reduce in size. This growth or reduction in size severs the connection to implants in a short time causing the implants to fail, and failing implants are likely painful if not dangerous for the wearer.

The best culprit is an evolutionary chain towards regeneration. People will need surgery for implants and as the population increases that has had to recover from more and more invasive surgery a selection process begins towards better biological regeneration. People with this better regeneration have less complications after receiving implants throughout their lives increasing their economic and social standing and chance to have strong babies themselves. Unfortunately this has caused a string of dead-end evolutions that included nerve regeneration, which does not have to grow back correctly. The surgery causes the nerves to undergo rapid growth to "repair" the damage not knowing that an implant has taken the place of the damage. This DNA string was first faulty near the end of people's lives where the higher risk of complications made it less noticeable but a trigger has caused it to start expressing itself in most of the population.