Once you become dependent on such a timesaving appliance, you'll need to know how to fix common problems. Avoid opening the microwave cabinet; doing so can be dangerous, and many manufacturers will void your warranty. See also How to Get Rid of Microwave Smells.
Make sure microwave is plugged in properly.
Check that door closes completely and its latch is aligned.
Check that interlock switch in door works properly (required for activating microwave).
Replace a blown fuse if it's accessible, or contact a service technician.
Inspect plastic coupling beneath tray.
Check ring and roller assembly for misalignment or debris.
Make sure that tray is sitting properly on turning mechanism.
If carousel motor is broken, contact a service technician.
Make sure there's no aluminum foil or metal trim on dishware.
Food residue or burned spots can cause electrical arcing. Clean chamber thoroughly. Replace wave-guide cover if burned or carbonized.
Sand away burn spots, and smooth rough metal edges.
Touch up spots with microwave interior paint if necessary.
If chamber has burn holes, contact a service technician.
If touch pad has gotten wet, then allow to dry for several days. Avoid spraying cleaning liquid near touch pad.
Check for insect infestation. Insects (cockroaches most frequently) will appear under clear surface of touch pad or under readout window; they like the warmth of the circuit boards.
Place window mesh over vent to keep insects out.
If you hear an unusual buzzing noise, the magnetron (the component that actually generates the microwaves) or other electronic module may be broken. This is not easily fixed; contact a service technician.