We need more information, please. What's she eating exactly, what's her age and breed? Is she standing or prone? Is she penned or free range?
Remove her to a small area by herself. You can try putting in one other buddy - the meekest of the rest of the flock - for company and encouragement to keep her eating.
Feed her her pellets only at the moment. try to encourage her to eat a treat of pellets, applesauce (nonsweetened is best), a little mashed/boiled egg yolk (freeze what you don't use for later use), a dab of yogurt, and some honey and/or karo mixed with water to a consistency that she'll eat. She needs to stay nourished with higher nutrition food, a probiotic (the yogurt), and water.
Do NOT medicate until you rule out other things like mites especially. Check her vent -t ell us if it's pasty. check around there especially for any bugs. they're very tiny, so check closely.
As LynneP mentioned, check her crop. Is it empty, full, mushy, smelly if you smell it from her mouth? Open her mouth - does she have any patches of anything funny there? A bad odor? Is she breathing normally, or open mouthed? Drooping wings? Bobbing tail?
How is her weight - feel her keel. Is it sharp? Or is she heavy? Is she maybe too heavy in the abdomen, or lumpy near her rear?
Saving her life requires that she's not dehydrated. Don't force her to drink, but do dip her beak in some water and see if she's interested. You can "spike" the water with a bit of karo, pedialyte, gatorade (only a small amount), or an ounce of organic apple cider vinegar to the gallon of water.