Dont even think about hitting the chuck key. ouch.
Find or make up a block of wood and use it against the jaws with the backgear engaged.
Move the backgear backwards manually and the chuck will undo itself, you may need a lever, like the bit of metal sticking up out of my lathe, you will need to adapt things to your own lathe layout.
Once you have the right setup a stuck jaw will never be anything more than an inconvenience.
Do it at right angles, like in the picture, so there are no weird stresses on the jaw, and vertically down into the lathe bed.
easy peasy lemon squeezy
Edited By Ady1 on 07/03/2012 23:54:01