I would suspect that she could have botulism from eating a toxin in a decayed animal or fish carcass, or from vegetation that has rotted without air getting to it. It can cause paralysis that can progressively spread from legs to wings, then to the neck. It can cause death within 12-24 hours, but if not much of the toxin is eaten, chickens who survive for 48 hours usually live with supportive care. Other causes could be lead poisoning, chemical poisoning, or possibly mareks disease, but Mareks usually affects one side more often. I would search for the thread "Go Team Tube Feeding," and tomorrow get some tube feeding equipment and KayTee baby bird feed in case you need to start crop feeding. I really hope you can help her recover from this. I would look around for anything another chicken could get into.