There are so many different chicen diseases its hard to tell
It might help to go to farm and tractor and get a packet of antibiotics , thou antibiotics will only prevent seconday infection and help aleviate bacterial infection, it may help ..it has a picture of a chicken on the front along with other livestock safe to use on with directions on the back for dosage...After you mix with water , you might need to use an eye dropper to giver her a little bit of the water to help her out a couple times a day, dont overload, and when she starts drinking again use for recomended number of days in drinking water.
The directions are a bit confusing, so dont stir too much into the water, it must be mixed every morning as it does not stay stable over 24 hours and turns an ugly color after that...