About all I know to try would be lubrication by spraying light oil down
inside the door in the lock area or getting it into a warm place if it is
very cold. If something has come loose in the door to prevent the lock from
opening, I would remove the door panel from the other door to see exactly
how the lock is arranged. Then I would try using a thin stiff wire or
"slim-jim" type of tool to trip the latch. A good locksmith might help a
lot, especially if he has the other door lock exposed to look at.

One other thing to try is to press or bang inward on the door as you are
trying to unlock it. Sometimes that will relieve the pressure on the lock
enough for it to work.

Good luck. Don Young