Switch off mains supply and pull cooker out.
Remove rear panel on grill side of cooker.
Behind the panel is a thermal fuse, on my cooker it's a brown (live) wire, it has a brown plastic sleeving covering the fuse and it's clipped on to the oven.
Unclip the thermal fuse, pull back the sleeving and check with a multimeter set on ohms range for continuity.
It should read 0 ohms. If it doesn't read anything it's blown and will need replacing.
You need to get the hobs off to replace as the fuse comes already wired and connects to the back of the clock display and to the power input on the rear panel of the cooker.
This is the most likely fault, otherwise there could be a problem with the clock.
If the thermal fuse is blown it will be because it's got too hot, this may be because the oven hasn't got enough ventilation to the rear, try pulling forward a little bit. Also if the glass top, when in the upright position is touching the wall, there may be heat building up causing the fault. Try getting a clip fixed to the wall that will hold the glass lid a bit away from the wall. You can get these in hardware stores.
Hope this helps.
Brian, August 2010