The dog is in pain, it will hold it's leg up, it will not put any weight on it, or it will put some weight on it but walk with a limp. If you suspect that your dog has injured it's leg, you need to take it to the vet and get an xray, to make sure that nothing is Broken. Say it is broken and you don't have it set and splinted by a vet immediately, it is not only extremely painful for the dog, but it will heal wrong, the vet will have to re-break your dogs leg, your dog will suffer even more unnecessary pain and that shouldn't have to happen. X-Rays are really not that expensive. Emergency Vets are open 24/7, so they are always available when you need them, especially after your regular vets office closes for the day. One of my Chihuahua named Gigi, broke her leg in two places. We took her to the vet immediately after it happened. She was in a Splint for 8 weeks. After she got the split taken off, it took about a month, for her to build up strength in her leg again and start using her muscles again and after that, you would never know that she had broken her leg in the first place. She was good as new, happy and running around having fun playing with my other Chihuahuas. What a little Trooper she was during this whole thing.The picture I uploaded is one of Gigi, right after we came home from the vet. She's on Pain Medication, and an Anti-Inflammatory, drowsy from the pain medication and a very long night and resting in her crate. She did have to go back in and get her Splint changed, not too long after, because the splint that the Emergency Vet put on her was too long and digging into, under her arm. The vet eventually got the splint the right length and size and that made things easier for her. 
