Sometimes chickens get sick for no apparent reason, and then they randomly improve all of a sudden - we had one who ate very little and complained in an annoying high-pitched whine while standing immobile underneath some tree. This went on for a couple of months. And then it molted and it was fine. Feathers grew back shinier than the young chickens'.

Now it eats as normal, although no longer lays eggs and moves around slower than the others. (However that's to be expected, as it's almost 4 years old.)
One of our first pair of chickens was sick, and recovered after we force-fed it garlic. (Seems chickens are like people and won't eat that stuff raw, lol.) I'm not quite sure if that's a good method or even a valid method, because it's only worked half the time and that could be how many of the illnesses clear up by themselves.
I hope your chicken will recover too - even if she doesn't, it's nice that you'll make her as comfortable as possible in her final days. We have only ever kept red sex-links and they don't seem to have it in them to live very long... It's probably a breeding for high production thing. Our first one died when it was 2 and a half, the second one at 4 and a half (it pretty much ate grass for the last year of its life though, so it was very healthy

) and our current oldest pair are just under 4.