I had this same problem with my oven!...
this is what i did:
switched it off and then back on from the mains (do this really quickly do not wait between turning it off then back on), then quickly pressed the centre timer button. Did this 2-3 times untill the display showed the set-time screen.
Hope that helps....!
Cheers
AK - just had this problem with my oven too apparantly its a common fault with this type of oven ive just done what you said as mine was displaying STOP and nothing was working and mine sprang back to life allowing me to use it again !!! your a star !!!
Chris
March 2010