Try putting some light oil, 3 in 1, on the key and put key in keyhole and work it back and forwards at the same time as pushing /pulling door - see if it frees up lock.
If it is a rim lock / latch key lock you can take off the latch 4screws and get to the lock and replace.
If it's a mortice lock ie set in the door then contact a locksmith.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix general d.i.y.