Hi, welcome to BYC. It could be a lot of things, but it is good that she is drinking water, that is most important. Have you ever dewormed your birds? It could be that she has worms. You could try getting some Valbazen, or Panacur suspension, and giving her some. Your vet might sell you some Panacur suspension (ours does)..he prescribes one cc per chicken per day for five to seven days. You can just shoot it right down her throat with a dropper (the big hole in her throat is the one that leads to her digestive innards) or carefullly ladle it into her lower beak a little bit at a time, and let her swallow it. It might be too late but it is worth trying at this point. If you can't find Valbazen or Panacur suspension, you can buy Safeguard for goats at the feed store too, but I am not sure what the proportions are for chickens; however, a lot of people use it, so search on here to find out. Safeguard is just Panacur. I would suggest deworming your other birds, too. Another option for the whole flock is to just get Strike 3 pelletized dewormer, available at feed stores, it does a good job and is easy as you just mix it with their feed and there's little to no withdrawal time. But use the Panacur or Valbazen for the sick one. For the other meds, you can't eat the eggs for two to three weeks after they are done with it. Worms can kills chickens easily, and as I said it might be too late, but you should try.
Continue keeping her in the warmth, that is important too, and in a quiet place. Our sick chickens love soft bread soaked in buttermilk, and buttermilk gives them some probiotics they need. Applesauce mixed with crumbles until it's very moist is another favorite of theirs. Or just applesauce, sometimes. Scrambled eggs, yogurt, cottage cheese, finely chopped grapes or banana, oatmeal, red jello, just damp crumbles are all good too. They do need fluids more than food so anything that contributes to that is good. You can also tube feed if she quits drinking altogether, but I wouldn't worry too much about doing that as long as she is drinking. if she won't eat from a dish try holding her in your lap and hand feeding her a little bit at a time, you can still use a small dish or a lid from a jar or something to hold the food if you want to, but sometimes they just like the extra attention too, and they will eat that way.
Lastly, it might not be worms, it could be something else, but worms are what I would suspect. Vets will do a fecal analysis too for you , if you can get enough to submit, and then you will know.
