First of all, treat for shock. You would be amazed how many times a chicken with a leg injury gets miraculously better after treating for shock which can often produce lameness all on its own. Give her a cup of tepid water with one teaspoon sugar stirred in.
Give her time to drink it, and then see how she is. Chances are she will have more strength and will try to stand. This is when you can make a more accurate appraisal of her possible injuries.
Note her stance. Is one leg splayed out to the side? Or is she holding it up under her belly? Feel both legs all along the length. Check for differences between them. This will tell you if something is out of place or if there is swelling and where the swelling is.
This article may be of some help.
https://poultrydvm.com/condition/slipped-tendon