Wooden garage door with mortice lock which will not open?

You do not describe the door but no matter - presumably the mortice lock has a tenon that fits into a mortice slot in an adjacent door - you might consider forcing the two doors apart - or if that's not possible - from the inside - chiselling out the timber around the mortice piece and - once the doors are open - fitting a better quality lock and making good the damage on the mortice side.

Given that you might intend to do as I suggest then there's little point in not trying to force the key just in case that works for you.

Good Luck...
December 2015