Has she been laying? Could she be egg bound? If you feel down near her vent and gently push in, can you feel an egg in there? If she hasn't laid for you in the last day or so, I'd get her into a warm bath and let her soak for awhile. If she's egg bound that will often help them relax the muscles and be able to release the egg.
If it's not that, does she feel thin when you feel her keel? Has she been eating for you? I know it might be hard to tell if you have her in with other birds, but now that she's separated keep an eye on how much she is eating/drinking for you. She could be sick with something and have gotten too weak to walk.
Have you checked her legs really well for any signs of injuries (swelling, etc) or can you find any injuries or blood hidden anywhere on her body under her feathers?
A vet is always the best thing to do in these situations so you can figure out exactly what's wrong and get her the proper help. My suggestions are just a preliminary check of things to try and look at right now before getting her to a vet. It's usually something serious when they can no longer walk and they need quick help. Very important that she is eating, so give her some of her favorite treats and see what she does.
