There could be so many things going wrong, from a pinched nerve to a neurological problem. No matter what the theoretical cause is, a dog that can't lift it's own weight is a dog that NEEDS vet care and quickly. I would highly suggest taking your pup into an emergency clinic ASAP. Keep us updated on how everything is going
"He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion."