What does lock jaw feel like?

Muscle spasm : Lock jaw can be an early sign of tetanus. The muscles of mastication go into spasm making it difficult to open your mouth.

Jaw is stuck: You can't open when you start from the closed position. It feels like there is something in the jaw joint preventing you from opening.

Pain/limited opening: I assume you're not talking about true lockjaw. A "locked jaw" is usually due to problems with the TMJ - jaw joint. You often have jaw pain, limited oral opening, history of clicking or popping of the jaw. If this is you, see TMj expert. Any dentist can be a TMJ expert with the proper training and experience. Most commonly, oral surgeons, prosthodontists, and orofacial pain specialists.

Limited jaw motion: In addition: the muscles may spasm, inability to chew, pain, swelling and tenderness.