If the inside light, cooling fan, and turntable are not all coming on when you hit "Start" to cook, then you likely have a bad top or bottom door switch, or that switch's mount is loose, misaligned, or broken.
See these help files:
http://031d26d.namesecurehost.com/mwd/safety.txt http://031d26d.namesecurehost.com/mwd/disassembly.txt http://031d26d.namesecurehost.com/mwd/doorsw.txt
If the inside light, cooling fan, and turntable are all coming on when you hit "Start" to cook, then chances are you have a failed high voltage diode or magnetron or a bad connection at the magnetron.
You can find helpful exploded view diagrams and order parts by entering your full model number from the tag on the oven here.
You have blown a fuse. Check inside the oven chamber and look for missing powder paint or signs of arcing, if so there is a paint to repair microwaves for this issue or else just dispose of the microwave. If the inside is good remove all the screws and remove the cover. Replace the fuse. If the magnetron was making louder noises prior then replace the magnetron. Since the fuse went the giant capacitor is likely bad. While the cabinet is open make sure to look for burned wires, check the continuity of thermal cutoffs, and check the main circuit-board for burned traces or components. The relays in most electronics are known for arcing (spot welding) and staying in the wrong position. Tap them hard with the back end of a screwdriver. During all of this use safe electrical practices by having the power cord unplugged. The capacitor could still have a high charge on it so be careful.
Sounds like you might have a problem with metal in the oven. Look inside the oven including the "ceiling" and see if the finish has worn off, leaving bare metal. If so, wash that area thoroughly, dry, sand lightly. Then apply some appliance paint that comes in tiny bottles and costs about $4.