It may be the fuse, which is usually located on the floor of the oven behind the control panel or between the door switches.
the fuse is good, it may be an open thermostat or thermal cutout (TCO)
/ thermal fuse on or near the magnetron or on top of the cavity / body
of the oven.
If it goes dead for a while during or after cooking
then comes back on, the magnetron is probably overheating and causing
the magnetron thermostat to open.
Then when it cools, it closes the circuit and allows power through again.
checking thermostats, if it has a hood fan thermostat, that should read
open, as opposed to the others, which should read closed.
If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.
If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.
it goes dead or blows the breaker (or GFI) when you plug it in or open or close
the door, then there's a problem with a door switch or door switch
If it's intermittent or random, it may be a bad
connection, usually on the control board or a loose fuse holder, or
even an intermittent fuse.
You should do a continuity test on
the fuse while it's in the holder (with the microwave unplugged, of
course) then turn the fuse by hand or take it out and put it back in,
then test it again.
If you remove the fuse, then press the meter
leads against the ends, it can allow internal contact to be made and
make a bad fuse appear to be good.
If you or someone you know
decide to look into it, we have critical safety information and
disassembly information at our site, and our link is at our listing here on FixYa
It will probably be a grey or white ceramic fuse. If you can't find one locally, try Smeg Service spares
.We're happy to help you and we'd appreciate your
thoughtful rating of our answer.