Easy...open it, remove all the dead geckos that are inside, look for broken capacitors and fuses....capacitors are very often the reason and easy to replace, easy to see, if they are mechanical deformed. Look for other burned parts inside.
If can't find something within an hour -->Trashbin
And believe the warnings of the others....Microwaves are way more dangerous than other broken devices, they can really bite if you don't know what you do.