It is possible she has Marek's disease- classic Marek's is progressive paralysis of the legs, and often tail and wings. They usually eat and drink well, but cannot walk. If you have ruled out broken bones or pelvis as the reason she cannot walk, then Marek's should be suspected. It can show up as a variety of syndromes, but the down bird that still eats and drinks well, is often Marek's. If you end up culling or if she gets worse suddenly and passes on her own- consider having her checked out by the state lab (call your local poultry extension or state vet for info on that). Then you will know for sure- as if you have Marek' s in your flock- you will want to make sure you either purchase only vaccinated chicks or start vaccinating them if you hatch your own.
There are LOTS of threads on Mareks' on this site. It comes up alot.