Emma,
Couple of things you can try, the doors keeps on the framework may just have to be filed out a little to allow the tounge to go home fully, it also could 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
December 2010