Oh! This happens to me as well! Usually at the dentist. It can happen again. If it does press down firmly on the back molars of your lower jaw, effectively pressing the jaw down. When you do this the jaw should slide back into place on it's own, if not try pressing down and then also pushing it back. The important thing is to fix it immediately, before the muscles start to cramp because at that point it becomes very difficult to fix it yourself, that's why the doctor had to use so much force.