Sounds like the monitor fuse opened and cut the power to the unit. There is a fuse and switch kit available (AP2929710) that includes a new switch and fuse. The switch is designed and a safety component and will blow the fuse if it isn't actuated properly by the door latches. Based on the information you provided, this is the direction I would start with. The fuse, BTW is a buss fuse available at most any hardware store. Just get the same size and rating.

Make sure the monitor switch (the center one) is opening and closing properly or you may blow another fuse. If you are lucky, it's just a bad fuse.

Yup, the monitor switch (AP2929710) that I suggested you check before replacing any fuses. When you start a cooking cycle, it takes about a second or two before the magnetron is turned on. When it does, if the monitor switch is still holding the monitor circuit closed, the fuse will blow and the machine will be dead. All three interlock switches must be closed in the proper sequence for the unit to work correctly. Most likely, the monitor switch has failed and needs to be replaced. Checking it's operation with a multimeter would be the best coarse of action.