I think there are two types of timer issues that stop the main oven from working - one is where the timer develops a fault and has to be replaced (relatively simple electronics that the spares people charge silly-money for) - the other is where the timer is accidently set to Auto (often when cleaning) and the oven just doesn't work...
When this happens on our oven I cycle the power so that the clock/timer unit flashes - then depress two buttons on the left and, while holding them in, use the ± buttons to set the time (any time will do) - and then the oven works.
Not all the timer units are the same - some have 5 buttons and others 6 - on some ovens you have to hold in buttons 2 & 3 from the left and on others buttons 1 & 2.
Hope this helps...
Good luck...
Peccavi, May 2009