She might have Coccidiosis. Coccidiosis causes sleepiness, droopiness, paleness, and lack of appetite. You might also notice bloody droppings. The treatment consists of Corid (either liquid or powder), given for five days in the water. The dosage for Corid powder is 3/4 teaspoon per gallon, and for liquid, it is two teaspoons per gallon.
Worms are another possiblity, but they don't usually cause problems so suddenly. It would not be a good idea to worm her if she is sick, as it is hard on a chicken's system. Still, it would be a good idea to get a wormer for future use. Safeguard is probably the best, but Valbazen is good too, though a little harder on the bird.
She might have external parasites, like lice and mites. Check around her vent, under her neck and back feathers, and under her wings. Treat with Sevin if you find them, or another mite product like Poultry Protector.
In the meantime, try to get her to eat something. Give her scrambled eggs, applesauce, yogurt, moistened chicken feed, etc. Make sure that she drinks enough too. Give her electrolytes and probiotics if you have them.
Hope this helps!