One of the great wastes in society is microwave ovens that suddenly stop working and get thrown away, when they could have been easily repaired. Most times, all that's needed is a new fuse, bought for $5 at the local hardware store. (First make sure that it isn't one of your house fuses that was tripped.) Over the past two weeks, I've saved two microwaves by inserting a new fuse. If you like fixing things, or know someone who does, this is a great way to save a good appliance.
Check out youtube videos about how to replace a fuse on your kind of microwave, and be sure to unplug the appliance before doing any work. In this photo, the new fuse is in the front, and the old fuse is in the microwave, near the top of the photo. Accessing it requires unscrewing the metal sheath that encloses the microwave. Be sure to remove the glass plate inside the microwave before turning it over! The sheath slips off but has metal grooves that need to be fit right when putting the sheath back on. Sometimes a special bit is needed to unscrew the sheath, but these are available at larger hardware stores. I pry the old fuse loose with a screwdriver, carefully avoiding disturbing anything else in the microwave. Then read the tiny print on the fuse to see whether it says 15A or 20A. Buy a new one, put it in, put the microwave back together, plug it in and see if it works. If it doesn't, then something else is wrong, but I've only had that happen once.