My jaw is locked what could be the cause?

Muscle Spasm : If your jaw muscles are in a state of "trismus" or spasm the jaw can lock and you may not be able to open your jaw properly. This is commonly due to an infected molar tooth (usually wisdom tooth), or due to heavy grinding. If you can open and the jaw locks where you can not close it unless the jaw is manipulated it is due to TMJ disease. In either case you should visit a dentist soon.

TMJ dysfunction: The muscles and ligaments that cause your jaw to move are not working smoothly. If you chew gum you should stop. Eat softer foods next 2-3 weeks. Apply moist heat to the joint area. Stress plays a big role in triggering TMJ problems. Find activities to help reduce your stress level. For acute episodes a physical therapist may be able to help. Your dentist can offer more detailed help.

Jaw locked: Temporomandibular joint dysfunction. May be due to grinding, clenching, muscle spasm, malocclusion, pernicious habits, but most probably disc out of position. Please see your Dentist.