Can you post the make and model number please.
Ok is the timer display on?
Is the time set or is it flashing?
No timer not on or flasing
Ok if the timer isn't working then the ovens wont work as they are power through the timer.
This is going to be either a thermal cut out at the back or the timer itself at fault.
What you need to do is pull the oven out and remove the back panel. Looking on the back you will see a thermal cut out. Its about the size of a 10p and has 2 wires going to it. You need to check the wires on there and with a multi meter you need to check for continuity. If faulty then you need to replace that and hopefully that will get the oven working.
But if the thermal cut outs are fine then its more then likely the timer at fault which will need to be replaced.
But just check the thermal cut outs first as its usually them at fault here.
Let me know how it goes.