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How old is your chicken? Does she lay eggs? What are you feeding her? Is it hot where you live? I would try to get her drinking some fluids. Water with a little sugar or electrolytes would be helpful. You can hold it up to her beak in a small cup. Mix some chicken feed with a lot of water, and mix until soft. Offer that and some bits of cooked egg. Separate her from others. A wire dog crate is good for that, and you can place her near water and food. Have you ever lost chickens or had an illness in the flock before?