Hi
Sounds like the door latch is broken. Its just happened to me.
I was going to buy a new dishwasher because I thought it would be simpler and engineers almost cost the same as a new unit anyway!!
I started to dig around and found the you can buy a latch replacement from domesticspares.co.uk. Latch Product Id: 6013.
The dishwasher door has about 12 screws on the inside. Take out the top 4 and the remove a couple from each side until you can see the latch (Don't mix the screws up, they are different), remove the 2 screws holding the latch and unplug the spade connectors. Get you new latch, plug in the spade connectors, screw it back into place (with the 2 screws) and then screw the door back together again.
The latch cost about £12 delived and even though I had never done anything like this before had it apart, replaced and working in about 15 minutes!!!
Good Luck
Stace
May 2009