Break the cap off, buy a new one. It's not an expensive thing to replace.
This happened on my car, it's not difficult ro rectify, but you need small fingerstry lubricating the locks with graphite powder (the right stuff, but 3in1 oil will do in an emergency).
If the door locks won't lock, then it could be the latches have disengaged. Removal of the door trim panels will be necessary to gain access to the inner door components.

Edited by Deathrow, 19 March 2010 - 11:08 PM.