How old is she?
Has she laid, if so when was the last time?
So she's now eating/drinking on her own?
Has there been any new additions to your flock?
Have you checked under her feet for any injuries?
What are you feeding?
Possible Mareks? Vitamin deficiency?
Where are you located? It helps when tagging others (time zones).
When folks come here and want to know what to do for a sick chicken, we need to ask a lot of questions to try to narrow down all the possible causes. Then we can recommend treatment.
Just as your doctor will start at the top of your body, ears, eyes, throat, we do that with a sick chicken, too. Look at the chicken's head, eyes, beak, even ears. Do you see anything unusual? Bubbling, swelling, drippy discharge? Anything in the ears such as foreign matter or waxy buildup? Open her mouth and see if you see any scummy or crusty lesions.
Check her crop. It will probably be somewhat full during the day after eating or drinking, but is it still full after she's slept all night? If so, we have a problem. Chickens can starve from a crop disorder.
If you can answer these questions and the ones just posted by @ChickNanny13 , we might get some ideas.
Have you seen a vet? I would recommend it. Might help to give age of hen and what you are feeding. Any past illnesses? Keep isolated with free choice food/water with poultry grit every day once over 8 weeks and oyster shell every day if over 15 weeks.