It's a BEKO double oven - it would not have hurt to have said that... but no matter.
You say "everything switches off" but give no further clue - does a breaker trip on the consumer unit? If everything switches off what do you do to get it working again? What is being heated at the time that power is lost?
I suspect that an element has become defective and is causing the earth leakage breaker to trip out - but this is just conjecture. Timers on ovens are usually for delayed cooking - the timer will switch things on - not off.
Good Luck...
April 2017