The thing to do first is to examine her thoroughly. Check for any injuries, swellings or abnormalities. Assess her body condition. Feel her keel bone.... does it feel reasonably covered or is it sharp with just skin covering it. Does she feel about normal weight or very light? Check her crop. Does it feel full or empty. If empty, offer some moist food... bread soaked in warm water or egg and see if she will eat. If it is full, withhold food overnight and check in the morning to see if it has gone down. Does she smell bad....from her mouth or her vent? What does her poop look like? When was the last time she laid an egg? Could she be egg bound? A warm bath in Epsom salts and massaging and lubricating her vent may help. If you suspect it is an egg, insert a lubricated finger gently into her vent to see if you can feel it. Giving her a crushed calcium tablet to eat may help her to pass it. Sometimes a soft shelled or shell less egg will cause them a good deal of discomfort until they manage to expel it, so that could be the problem. It's important to dry her well after her bath to ensure she doesn't get chilled. A hair dryer on warm setting works well.
Many people will also suggest checking her over for lice and mites but in my experience, these parasites will take advantage of a sick chicken rather than be the cause of the illness. That said, a severe mite infestation will cause anaemia, but you would expect to see other chickens also looking sick in that circumstance and if you are in winter and a cold climate, mites are less likely to be a problem. .
Antibiotics should only be used when you know or are reasonably sure there is a bacterial infection and then it is important to get the correct antibiotic. Ruling out all other causes first is the appropriate action.
You may find that being brought into the warmth and given a bath and blow dry and something to eat/drink will revive her, but worth keeping her in overnight and checking poop and crop emptying even if she perks up.
Good luck with her
Barbara
