Microwaves are on a dedicated circuit with nothing else using that power lead, so tracking that specific power leg should be easy. Find where it's plugged in (maybe under your sink somewhere). Unplug it from there and use an outside extension cord to test the microwave. If it works then the issue has to be in the dedicated line from the the circuit breaker...or the breaker itself.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie