you will usually find at least one interlock switch on the door latch, and one or more thermal switches in series with the transformer primary. the door interlock can get out of position and not close the contacts when the door is closed, the thermal switches can fail open circuit. if the door interlock acts indirectly by energizing a relay, the interlock relay can fail.
KISS wrote "There is usually a mains fuse and a NC interlock switch across the mains after the fuse." (??????)
such an interlock would blow the fuse every time you open the door..... most common is a microswitch that drives an interlock relay on the control board, since microswitches that would actually be in the primary circuit wouldn't last long, or they would be expensive because they would need to be rated at 10A.
the most common failures in the primary circuit that keep the oven from heating, are:
thermal switch open no power to the transformer, but seems to be operating normally
interlock switch failed, or out of position, depending on which switch, may not get primary power to transformer, or controls act as if the door is open
control board failure, usually bad relay contacts if everything seems to be working normally except the transformer never gets primary power. you need to listen carefully to the relays on the board.
secondary circuit failures:
magnetron, usually filament goes open circuit transformer hums, and cap is charged, but no heating.
cap shorted, or shorted to ground. usually blows main fuse.
shorted diode, again, usually blows the fuse
open diode, cap never charges
shorted turn in transformer, blows the fuse
open primary in transformer. power gets applied to primary, but transformer doesn't hum