You will need a model number tro be able to find the parts.
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Could be the control board/clock/timer but there are also other possible causes.

First you have to check the power. The elements require the full 240 volts but the clock and oven light only need half the line (120 volts).

Try flipping the breaker off/on slowly a couple times. Sometimes you can loose half the line without actually tripping the breaker.
Check the voltage at the plug
L1 to L2 should be 240 volts
L1 to Neutral and L2 to Neutral, both should be 120 volts.
If OK
Unplug the unit and check the wires at the terminal strip in the machine to make sure none are loose or burned out
If OK
Check the power at the terminal strip.
Do this with the heater off and on.
[COLOR="Red"]Be careful as 240 volts is lethal !!![/COLOR]

Does the oven say it is ready or does it stay in preheat?
If it says it is ready then it could be an oven sensor problem.

Also some units have a thermal cutoff in the element circuit.
I would need a model number so I can search for a wiring diagram of this uniot.