Its likley to be a burnt out magnatron (thats the part that heats the food up) if its burnt out then a fix would cost as much as a new microwave would there is nothing you yourself can do to fix it and just trying to could kill you so to be on the safe side drop it in the shop for repair or bump it and buy a nice new one
Answers & Comments
Magnetron is $130
You can change it and if built it, you cannot buy new microwave for $130
You can get shocked with HighVoltage so if you can't use a VOLTMETER, DO NOT ATTEMPT
repairclinc.com
Its likley to be a burnt out magnatron (thats the part that heats the food up) if its burnt out then a fix would cost as much as a new microwave would there is nothing you yourself can do to fix it and just trying to could kill you so to be on the safe side drop it in the shop for repair or bump it and buy a nice new one