This happens on my oven - when the power goes off and comes back on again.
I depress and hold in the two buttons on the left of the clock and use the plus and minus buttons to set the time - any time will do- the clock stops flashing and the oven then works - until somebody accidentally sets the auto timer with over enthusiastic cleaning.
Your oven may be different - but set the time.
Good luck...
Heston Blumenthal
September 2014