Sounds like the little cam's come loose from the lock.
You'll need to break in to the box (flat bladed screw driver, butter knife etc) and unscrew the lock cover. After you've collected all the bits that spring out
you can reassemble and tighten up the little nut that holds the cam on. Mine was fairly munched up (it's only plastic) but it still holds and works.
I did consider making another in brass, but never got round to it.
A common cause for this is having the box locked to the bike, then trying to remove it with the key. There's a little knob inside next to the catch that says 'open'/'closed' - if it's set to close, it stops you turning the key anticlockwise to remove the box. Of course, forcing it was pull the cam off