Joe,
Had this problem a couple of weeks ago, it will be the gearbox mechanism that needs replacing. Bad news is to get a locksmith out to do this job, expect to pay around 150 to 200 pounds. If you want to try replacing it yourself, you need to locate the maker and model number, which can be found on the metal strip that runs down the frame of the door. There are plenty of companies online, that sell these replacement parts.
Good Luck
Brian L, November 2010
