How old is she, and was she vaccinated for Mareks disease? Mareks is a virus that can cause paralysis in one or both legs or wings, and a limp or twisted neck. She also could have suffered a severe leg injury. Keeping her penned up near food and water, and adding some poultry vitamins to her water would good so that you could observe her. Hopefully it isn't Mareks, but if you lose her, it would be good to get a necropsy done by your state vet to look for a cause. Keep us updated on her condition if you will.
The risk with bringing in a new bird, is that it could have been a carrier for Mareks disease. For instance if there had been Mareks in the flock before, the new one could have been vaccinated or immune to the disease, but still be a carrier. Your chickens can become symptomatic within 3 to 4 weeks of exposure. Mareks can cause paralysis, eye changes, and tumors in the body. Hopefully, it is not Mareks, but if you should lose her, a necropsy would give you some answers. I do hope she recovers. Here are some links to state vets and others who perform necropsies:
Here are some links to read about Mareks disease: