Don,
Try the following:
a. Unlock it by using key. Well, duh!
b. Spray some lubricant in there (just a little WD-40 or something equivalent.) Then try it again.
c. Insert several finger nails along the top and pull/jerk down
d. Borrow a dinner knife from your wife. Slide it in to trip the mechanism. (No. Don't ask her. Explain later when she asks you if you have seen it. Straighten it before you bring it back.)
e. Use a screwdriver. (Be careful because the shank is bigger than the knife.)
f. Use a pry-bar. (This is a last resort effort (from the front) and you may want to check the availability of another door before attempting this.)
g. Go ahead and tear the box out from the rear. And next time check that the lock is working properly before you lock it.
You're welcome,
Charlie