There is a plastic cap on top of your agitator inside there are plastic "dawgs" that are rigid like a gear which grips the shaft when agitating. anyway they may have been bumped loose.there should be three or four in there. use one of those links that old turkey put in his reply to show you how to replace them.

Although I agree with not touching anything inside the unit a fuse replacement is an easy fix. on most microwaves all you do is remove the cabinet and you should see the fuse in the open. use a fuse removal tool and replace it with the same voltage and amp fuse. DON'T TOUCH ANYTHING ELSE!!!! The capacitor inside the has enough voltage up to 300 volts to deliver a good zap. The magnatron from what I understand will kill you.