It is very rare that both ovens on a cooker should fail at the same time. More often than not the problem is resolved simply.
If you are finding that both of your ovens are not responding, first you need to check if the clock is illuminated. This will indicate whether power is reaching the appliance.
If the clock is flashing, then this would normally indicate that power has been terminated and re-established with the appliance. In this mode, the appliance will not function until the time has been set on the clock.
Other times the clock my have been set to auto-cook mode, where the oven will start cooking and finish cooking at a specific time. While this mode is set, it normal for both ovens to seem like they have stopped operating. Check the timer to see if 'Auto' is illuminated, if it is you need to revert it to manual cooking mode in order to operate your oven again.
If you still having problems after this point, it is likely that your timer may need to be replaced. Bear in mind that, essentially, the timer in your oven or cooker is really just a big mains switch so if it fails in a position where the power to the oven is off you will have a dead oven as well as the clock.