This happens to my front door lock - the brass barrel of the lock mechanism has a chrome mini-handle that works loose over time. You will need a screwdriver and the key.
Undo two small screws thst hold the door latch. They are on the side above and below the latch itself. Wriggle the body of the latch and take it off the door.
You will see a metal plate with two screws that hold the lock barrel in place. One may be covered up by a piece of mechanism - put the key in and give it a twist and the hidden screw will come into view. Tighten these two long screws to pull the lock barrrel in. Put back the latching mechanism and replace the two small screws that hold it in place. Test with the key. If too tight loosen the screws holding the barrel a quater turn.
Peccavi, November 2008