Jaw pain just under the ear on the left side. Range of motion is extremely limited. What is the matter?

Infection. If the problem is new, an infected, impacted third molar (wisdom tooth) may present this way.
Need evaluation. It is hard to say exactly what is going on. Your jaw joint is next to the ear and you inability to open points to a problem here. D have you had trauma or been hit recently? Does your jaw click or pop? How long has this been going on? These are some of the questions that need to be answered before a diagnosis can made and treatment recommended. Find a dentist well trained in treating tmj.