Was she vaccinated for Mareks disease? In what part of the country (or world) do you live? She sounds like she is having aome neurological issues, and Mareks, among other problems could be possible. Some viruses, lead poisoning, botulism, and other illnesses could be a problem. How old is she? Try to get her to drink some water with vitamins and electrolytes in it. SaveAChick is one of many brands, and homemade electrolytes are also good. Buttermilk added to chicken feed, or to bread crumbs with a little raw egg mixed in with usually get chickens to eat. Here is a good thread about Mareks disease:
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq