I went through this last year with my dog. I thought he was going to be paralyzed forever. After a poor diagnosis and treatment by a newbie vet in the office for over a week (who is no longer there) different vet finally gave him a cortisone injection directly on the location and he was walking 48 hours later. He has a calcium deposit or something in his back (photo of x-ray below). When it gets inflamed, it pushes the disc up into the spinal cord causing paralysis. After the inflammation subsides, the disc goes back where it belongs and he's fine.
That was the worst he's ever been but I can tell when it is bothering him so I make sure to keep him quiet and don't let him jump on anything and our walks are slow and steady.
Good luck and if it doesn't get better, there are wheelchairs out there for dogs and they seem to adapt pretty fast.
Laura