Contacts to the door switch, are the most common reason for this problem.
The diode and capacitors can also be tested. The capacitor must be allowed
discharge before testing, or you risk to get electrocuted, even if
power is off. Capacitors can get discharged by shorting their two
Replace the magnetron if the checks and all of the high voltage component tests are good, but the unit still does not heat.
All locations, and description of this parts can be located using the diagram here: Counter top
Parts must be tested to find out what is wrong.
Remember that microwaves are potentially dangerous devices if not repaired properly, the job must be done by a competent person.
A microwave energy leakage test must always be performed when the oven is serviced for any reason.
Regarding parts, contact the seller here to purchase parts. (here in UK).