How old is your hen? Was she vaccinated for Mareks disease? For the time being, I would place her in a crate or pen where she will be forced to rest her leg. Add some poultry vitamins or BComplex to her water. Can you post a picture of her standing? Can she move her toes if you touch them?
Mareks is usually progressive. In other words, if you have been watching your flock, you would have seen her get worse and worse over time. If this just turned up in the last day or so, I would suspect injury of some sort.
Give her an over all exam to make sure she doesn't have any external injuries. Do you keep a rooster? If so, look under her wings, behind the neck, abdomen too where the spurs might have punctured her skin. Feel her feet, foot joints, hocks and up to the hip. Any heat, redness or swelling? If she broke something, it would definitely be hanging. You can usually feel dislocation too. Compare her hock and hip joint with the opposing one. They should feel the same. A dislocation will be sticking out pretty badly.
If there isn't any swelling,heat, redness or inflammation, it is possible it is Mareks. Look into this birds eyes. Many time with Mareks the pupils are not round, but oval or out of round. Compare eyes with other birds eyes.
But for now, I would confine her for possible contagions, and also if this is a simple sprain or strain that she might heal by not using her legs as much. Keep her in an area where food and water are close at hand so she barely has to move to get them. Sprains can take 4 to 6 weeks to heal. If this is Mareks, she will go down hill pretty fast in this amount of time, slowly becoming paralyzed all throughout her body, eventually no doubt leading to death.
Sometimes you have to make decisions for them. If she is limping or hurt in any way, she should be confined until it heals. If they continue to push a strain, it could turn into a torn ligament and these don't ever heal. Generally they end up having to be put down.
So give her a good assessment and do what you think is good for her.