I have an Ariston LS609 which had this problem. The problem was the catch was broken inside the control panel. Pull the dishwasher out, unplug it. Look on the back of the top panel and there are 2 screws. Undo these then slide the top towards the front of the dishwasher and it will lift off. In the middle front top there is a small plastic arm with one nut, undo this then the door will open. Then ascertain if you need a new latch mechanism. My dishwasher is about 10 years old, so I didn't want to waste money on it, so I repaired it with some longer screws which could cut deeper into the plastic panel.
Gordon C. Melbourne, Australia, September 2008