If a dog has dislocated it's leg then there will be swelling at the leg will usually be laying at an awkward angle.
Limping can also occur when the dog has broken it's leg or has dislocated it's leg.
If you think the dog has broken it's leg or dislocated it's leg then you should take the dog to the vet.
Sudden limping in dogs can be caused by several things such as thorn in the paw, needle in the paw etc.
Some other things that can cause sudden limping in dogs are sprained ankle, sprained paw joint and even arthritis and a variety of other conditions.
If your dog is limping on his front leg it could be that the dog has a thorn, sticker etc in it's paw and is limping to keep from feeling the pain.
If the dog has it's paw nearly on the ground but still limping then the dog may have sprained it's ankle or may be having arthritis in it's leg.
Some other things and conditions that can cause a dog to limp on it's front leg or other legs are things such as Osteoarthritis, hip dysplasia, elbow dysplasia, patellar luxation, ligament disease, intervertebral disk disease, and osteochondritis dissecans.
If the dog has no thorns or anything else in it's paw then it could be any of the other issues and you should probably take the dog to the vet especially if the limping lasts longer than a few days or so.