I would suspect the thermal fuse / thermal cutout or thermostat (whichever it uses) is open. It is located on or above the magnetron.
When the mag gets too hot, this device protects it from destructive overheating by cutting power to the oven. Hopefully it did its job!!
If you have a Sharp R308 (the heading you posted in) then it will be a thermal cutout.
It should read virtually zero ohms or full continuity, depending on how you check it.
If you or someone you know wants to look into this, we have *critical* safety, disassembly, and troubleshooting info at our site, and our link is at our listing here on FixYa:
http://tinyurl.com/yzjozkYou can usually find exploded views and by entering your FULL model number here:
http://www.searspartsdirect.comYou should also check the internal fuse, just to be sure.
Please reply with your brand and model number if you need further help or if yours is a different model.
We're happy to help and we appreciate your thoughtful rating of our answer.William E. Miller, AS-EET
[email protected]http://www.microwavedisplay.com