Chances are the door is out of alignment and you have been forcing it to lock. If so the internal mechanism has probably broken and will need opening & replacing. This can be rather expensive and should be carried out by a reputable locksmith, get a price beforehand though. If the key is going round & round it is possibly a cylinder problem which should be cheaper. Hope this helps. If you need any further help, just ask. I am a locksmith and have solved many of these problems.
Dave The Lock, September 2005
