Before you replace the entire Defrost Control Board though, be sure to individually test both the defrost heater and the defrost thermometer. You can check this out by turning the defrost cycle timer dial slowly to the defrost cycle. This should result in the compressor turning off and the heater turning on. If this does not happen it is most likely the proper time to replace it with a new defrost timer.
If the door has been closed for 5 minutes or longer it will not start until the door is open and closed again. This is a safety feature that stops the microwave from being started with no food or drink in it. Also, the control lock, or the countdown timer can be active which needs to be cleared.