I'm no expert but it could be as simple as an internal fuse, in which case IF you know what to look for & how to test safely, unplug the unit, undo the cover and check for all obvious fuses internally. Replace any that prove faulty with identical and check the units' function. If it's not an internal fuse, as everything else is working then it's probably the magnetron (produces the actual microwaves), if so it's not worth repairing - go buy a new one instead. Hope that's useful.
Graham H, January 2009