It could be an injury you didn't notice or a stroke. Calcium deficiency makes frogs have trouble moving their muscles, but it's unlikely one would work and the other wouldn't. Or maybe it somehow has something to do with a fungal infection.

If it's the first possibility I don't know that there's anything you can do.

There may be other causes for this I don't know of: I'm not a vet. Hopefully others will chime in.