Could be a few things such as Marke's disease or a Vit. B 12 deficiency. Get some chicken electrolyte solution from the feed store and put it in the water. Or add Vit. B (liquid form) in the water, a few drops. This may help. it helped one of my hens, but it took 2 weeks before she could walk again, but she was fine after that. Now I give my hens Vit B 12 in their water once a week.
I appreciate the help though when my mother closed the coop she did not check to make sure she was in there which I always do. So she got locked out and froze to death. And she knows I check so my little 2yr old baby died.