The following scenario occurs:

1. Alice takes the Ready action to do something at a later time (the specifics don't matter).


2. An enemy casts Hold Person on her, she fails the save and is now Paralyzed, and thus incapacitated and therefore she cannot take actions, or reactions.


3. By some means the enemy concentrating on Hold Person loses concentration.


4. Whatever Alice wanted to react to, occurs.

Disclaimer: Alice's turn has not come up again, and so she has not lost her readied action.

Can Alice still use her reaction to take the action she had readied, even though she was incapacitated after she had readied it?

Condition changes have no effect on a Readied Action unless it is a spell

The Ready action just says that you are able to take an action using your reaction at some point before the start of your next turn.

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn. (PHB 193)

There is nothing about it that would be canceled by becoming incapacitated.

However, you must become free from your incapacitation before the start of your next turn, otherwise you lose your Readied action anyways since you can only Ready an action until that point.

The only exception would be if you Readied a spell since being incapacitated forces you to lose concentration which also means you would lose the Readied spell.

To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration (explained in chapter 10). If your concentration is broken, the spell dissipates without taking effect.

Alice can use her Readied Action as a Reaction if, and only if, the Incapacitation is removed before the start of her turn

The rules for the Ready Action specifically stipulate that it allows the user to use their Reaction "before the start of their next turn":

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.

—Ready, Player's Handbook, pg. 193

So if between being paralyzed and the start of their turn, the paralysis is removed, then they would be permitted to carry out their Readied Action.

The only major exception is if you were holding Concentration on a readied spell, since incapacitation breaks concentration:

Normal activity, such as moving and attacking, doesn't interfere with concentration. The following factors can break concentration:

[...]

• Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die.

—Concentration, Player's Handbook, pg. 203

So because the Ready Action requires you to maintain concentration if you are using your Ready Action to cast a spell, you would lose the spell, and therefore your Readied Action, from being paralyzed.

Yes, Alice is good to go

As you state, being incapacitated will prevent Alice from using her reaction. However, acquisition, and subsequent loss, of the incapacitated condition doesn't interfere with her ability to have a reaction once she's no longer incapacitated.

I don't think this is explicitly stated in the rules. Rather, there is no statement saying that any condition change causes you to lose a readied action.