Never had that happen, but maybe you could knock it loose with a hammer and screwdriver held at an angle on the rim.
Big locking plier or jar lid remover. Just some ideas off the top of my head. Good luck.
Is it the cap which is stuck or the lock which won't turn?
If it's the lock try a little WD40 or other release oil in through the key hole, put the key in and out several times to help the oil penetrate the cylinder and turn it GENTLY so as not to snap the key in the lock.