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.