problem

There is a fuse inside the unit that may need replacing. A web search of your make and model of microwave will direct you to where it is.

Inside the microwave is a fuse...they blow when the microwave is faulty.

Usually cheaper to replace microwave vs attempts to repair

You can find a repair manual on-line via goggle search of make/model if you care to attempt replacement of fuse and/or repair

Run extension cord from known good power source to microwave. Plug in microwave. Do you get a light (if equipped) when door is open? If yes or no light installed then>
Try to Start microwave.
Microwave starts yes.

Then plug a good lamp into the the receptacle for microwave. Report results.

Microwave does not start then replace/repair microwave

Our microwave was just replaced under warranty by FR. It worked fine and then it didn't, but the last use it got very hot. DH shut off the circuit until we could get home and check it out. If you do need to replace yours, be aware that the vented trim pieces don't fit all microwaves. Both of ours were Magic Chef, but they had to include a new trim piece.

Good testing idea above. Any chance yours is still under warranty????