Just in cased you haven't got the manual with you it states:
Are location and ventilation satisfactory?
Is the appliance level?
Is there a current in the wall socket and is the connection suitable for the appliance?
Is the mains power cable damaged?
For mains operation, is the electric thermostat swiched on? For gas operation, was the safety valve knob pressed down long enough?
Is the thermostat knob set towards the maximum position?
Is the gas cylinder or the pressure regulator valve open?
Is there any gas in the cylinder? (If by shaking, no liquid movement can be detected, then the cylinder is empty.)
Are by any chance two different power sources connected (such as gas or electricity)?
Was warm food placed in the unit?
Was a large quantity of food put in at one time?
Hope that helps, if not I can investigate further.