Have you checked her over for mites- they can be debilitating . Is she laying yet? Any chance she is eggbound? Have you checked her over for any injuries, that could be hidden under a puff of feathers. Has anything changed around the time she start showing illness? Different feed?
Drumstick has some great ideas. Another person on here had the same problem. His chicken wouldn't eat either. I suggested cutting up a hot dog in small pieces. He wrote back and said his chicken started eating it right up. so maybe you could give that a try. I know my chickens go crazy over them. Good luck. Merry Christmas and a Happy New Year.