I can't open my mouth all the way (jaw is locked) for some reason. what can i do?

See your dentist: Do not try to force your mouth open as you may potentially create a more significant problem. You need to see your dentist asap for evaluation and recommendations for treatment.

Disc: The disc between the upper and lower bones in your jaw joint is probably out of position. DO NOT try to force your jaw open. First, call your Dentist for an emergency appointment. Then, take Ibuprofen/motrin/advil as directed on the package, and apply cold compresses. This will help keep swelling down until the Dentist can see you.