I don't know what's wrong
Injuries are common with chicks, and if her leg is sprained, she may need to rest it for a period of time. Does she hold the leg out or hold it up if you try to stand her up? Can you look over the legs and foot joints to see if any are swollen? I would leave her with the other bird in a small pen with food/water as long as the other is not trampling or picking on her. Or they could be placed in separate pens to see each other. B Complex vitamins or poultry vitamin powder can be added to the water. Pictures always help, and if you need help on splinting, just post again.