Dog doesn't move one leg Help

[Queenofdapiggies]

Ok so I was playing with my younger sister and we slept in the living room. That night my dog was perfectly fine. The next morning when I woke up my sister was on the computer and I was on the couch my dog was sitting in his dog bed. After a few minutes my dog tried to get up but I realized that one of his back legs was sticking straight out and he fell back to sitting he wasn't whining or whimpering so my mom took him to the vet he has to stay the night and they say he's not doing better. What could be wrong with his leg?

[ginger899]

I don't know for sure, but I am certain the vet will find out.
It is possible it could be a compressed spinal nerve caused by something in a disk (a disk prolapse maybe?) But I can't tell, as I am not a trained vet, and at this distance over the internet.
Has the vet done a scan or an X-ray? Sometimes an MRI scan is needed to be able to see what might be happening to the spinal nerves.
You (or your Mom)needs to speak to the vet to find out what tests they have done and what they suspect.