How old is she? Have you ever had Marek's confirmed in your flock? How many birds do you have in how much space?
First thing is to try supplementing vitamins. If she's been kept from food that may very well be it. I would crate her, get her eating and drinking, give her a super B complex vitamin once a day (human ones - regular not time release). You can give her some cooled scrambled egg, see if she'll eat that. If she's being kept from food and water then you may need to add more of both, to give her more opportunity. The higher ranking birds can only guard one at a time. While she's crated I would monitor droppings, make sure they are normal. Make sure crop is functioning normally, check her over for lice/mites or any wounds or injuries. Just a general health check looking for anything.