Firstly I'm sorry to hear about your problem, this job is a bit of a fiddle but if you take care is not too difficult.
a. Firstly disconnect the appliance from the water and electric, b. Remove the top cover. c. Taking a torch, look down inside the appliance and you can see where the door catch comes through the outside panel and pushes the door lock (called an interlock) sideways. d. Taking care slide your hand down inside and see if you can push the door catch sideways to release the door. e. If you can't then you will need to break off the interlock (its a bit of a fiddle but eventually you will release the door) f. Once the door is open, release the door seal (rubber) retainer from the door seal, pull the seal out of the way and you will have access to the defective interlock, which is held in place by two screws. Note the way that the three cables are connected to the interlock. Go to your local appliance centre and buy a new interlock, transfer the cables across to the new item and replace the remainder of the parts in the reverse order. This is a common fault with these appliances ~ should cost you about £10 or so for the interlock.