Start by disconnecting the mains supply and wait three minutes. If the door STILL doesn't open after that then you have a genuine jammed door problem! You can try pushing the door shut - tapping the door catch area - and try again to open. If that doesn't free it you are going to have to get access to the back of the catch, probably by removing the top cover of the machine and reaching down to the door lock assembly. That's the best help I can give. Good luck.
Richard
December 2008