these locking systems sometimes have a method where the locking bar at the top and bottom are on a small plate about 2mm thick by about 15mm wide and 50mm long with teeth along the long edges, these teeth fit into an opposing bar that are connected to the handle system, this is so they can be slackened off and adjusted by sliding the bolt up and down the bar.....if this mechanism works loose then the two parts will separate and not function......when the door is open, you should see a set off screws along the locking system and i would think the ones at the top have worked loose.....its just a case off tightening all the screws and lubricating the mechanism........when these doors are made they are just long lengths of profile, cut, mitred, metal strengtheners added and welded together to form the door at the correct size, then all the hardware is added afterwards like hinges, locking systems etc......you never need to replace a door because of a failed lock or part of a lock