I'd get your dog to the vet as soon as possible, some disease or problems can kick in very quickly and suddenly. If you don't want to bring it to the vet then maybe just call the vet and describe whats happening to your dog to them and see if they think it is appropriate for you to take a trip to the vets. Just don't wait too long.
All of a sudden my dog can’t stand up. What do I do?!
The causes can range from infection, to trauma, to disc disease.