Hopefully the WD40 will work. You can probably open the door from inside.
If it doesn't work, there is a little spring that costs about £1.50 to replace in the mechanism. Taking it apart and and putting it together is a bloody nightmare though. If it turns out to be this then keep this thread open on LZ 'cus there's a few of us that have had to do it. The spring is available on eBay.
Search facility - top right - should bring up previous threads for you to pick from