Fixing common problems with time saving appliances we dependent on is time well spent. Avoid opening the microwave cabinet; doing so can be dangerous, and many manufacturers will void your warranty.
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.
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.
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.