Have you added any new birds recently to your flock? Has she been laying eggs up till now? Have you wormed her recently? I would open the eye and inspect it for any foreign object or drainage, and you could try some lubricating eye drops, saline, or Vetericyn eye wash or eye gel. Look at the eye and compare the color and the pupil to the other eye. Valbazen 1/2 ml orally to each chicken and repeated in 10 days, is a good wormer. Is she having diarrhea or blood in her droppings? Has she lost weight in her breastbone area, or is her lower belly full or swollen? Check her crop for impaction and make sure that it is emptying overnight. And lastly, check her skin especially under her vent area for tiny bugs (lice or mites.)
