If others start showing symptoms, it very well could be Mareks or other communicable disease that has neurological symtoms. I have read that a hallmark sign of Mareks is paralysis causing one leg to be under the bird, and one leg extended behind the bird. Like Keith said though, there are so many illnesses that can manifest so similarly in chickens and it is so hard to pinpoint what it is without blood tests, necropsies, etc.
I've heard that moldy feed can make a chicken ill, but I've not had experienced it. They get into anything and everything, especially the free ranging ones though. I've had old timers tell me that it is dang near impossible to poison a chicken and I believe that to be true. They are tough little critters. Keeping her hydrated and well despite her paralysis will tell you if it is something temporary she will recover from or not. Best of luck with her!