Sorry to hear about your hen.
It could be that she just injured it, slipped a tendon or perhaps broke it. Chickens heal from breaks pretty well, though you should splint it if this is the case.
She could be egg bound. Make sure that isn't the problem by feeling her abdomen for any eggs. Egg binding can cause leg problems and can quickly lead to death.
Other problems that involve the legs include partial paralysis, which can be caused by viruses, dangerous diseases like marek's, or an injury to the spinal cord.
I've had chickens with limps from jumping far distances or arguing and they heal up in a few days.
Keep a close eye on her and make sure she is warm, eating, and drinking.
Hope she gets better soon!