I have have the same problem. After doing a quick search it appears to be a common fault.
You can try to switch it off from the mains for 30 mins, or others say remove all food and switch off for 48hrs. So something in between these two. Then when you turn it back on it may work again.
Otherwise it may be a thermostat or the controling PCB (circuit board).
Hope this helps
Glen
February 2008