I dislocated my shoulder and the muscle deteriorated and now I don't have a muscle in my left shoulder. Is it possible to get my shoulder muscle back?

Muscle wasting. I am concerned that you may have had a nerve injury secondary to your dislocation. This is usually the cause of posttraumatic muscle deterioration. You will need to have a nerve study (emg/ncs) to determine your nerve function then a decision can be made about the future health of your muscle.
Yes through PT. Physical therapy is an important intervention. The good news for dislocations is that this is one disease/event which occurs less often as we get older. The ligaments stiffen and make it more difficult for the bones to dislocate.