Do not drill the lock, it is most likely a latch fault. First be sure nothing is jammed inbetween the door and the door jamb. Open and close the lock from the inside, watch which lever is moving at the actual latch. Remember the lever, and disconnect the linkage from the latch from the lock, now pull on the handle from the inside and watch what portion of the latch moves, remember the arm it's attached too and disconnect it. Do the same for the outer handle, and key lock and disconnect them. Now with no linkages attached to the latch, manually with a flat screwdriver, move them in the direction they would naturally move to unlock and open the door. If the levers on the latch, seem to not move all the way than put pressure on them, until the door opens. This would verify a faulty latch, if the door opens easily with all the linkages off, than one of the components attached to the linkages is binding, not letting the door latch completely unlock.
-Rip
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