If you do not know what you are doing call a repairman. Anything you do can be dangerous due to electrics and sharp edges etc. Yiou have been WARNED that anything you do is at your own risk and PERIL.
Could be anyone of a number of problems. First unplug from mains. Then plug in and try again. Next there is a sequence of lights that comes up and you need to search the Internet to find out what buttons to press depending on the number of lights showing. If this is not the problem then you may need to take the front fascia off, and then the front panel at the bottom of the machine and press the button in the middle of the sensor on the left side behind the heating element. You will need a torch and mirror to see where it is. I don't know if this has to be done with the machine plugged in to the mains or not. But beware of electrocuting yourself! If none of the above fix it then again unplug from mains and try taking the machine out and lowering it down to remove all water as the float mechanism may be switched on. If noe of the above work then you will need a repairman as the computer circuit board (the brains) may be faulty. A replacement panel is over 120 pounds. If that's the problem then because it is almost half the cost of the machine and your machine is probably out of warranty or you would not be fixing it yourself you are best advised to buy a new one but go for a Zanussi or Electrolux (these two makes Z & E are the same machine just different branding and pricing). The Diplomat is probably an Indesit and they appear to have a poor record for reliability given what I have read on the Internet.
Richard
April 2006
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix dishwashers.