There is electrical problem in your microwave heating circuit. This is a fire hazard and you should stop using this microwave and unplug from the outlet. This is not something you can fix and the fuse does not protect your system anymore. The best choice is to purchase a new one and throw this away.
There are fuses inside. The big thing is to figure out why the fuse went bad. either something shorted and blew the fuse, or it got hot and went bad due to bad connections at fuse holder. or maybe a bad door switch blew the fuse. I have also had fuses just get old and tired., and just needed to be replaced.
