Make sure that the program controller is working if not checkpower - step down supply, the pads are responding- if not faulty pads- or thedoor switches, if the switch contact are not good then it will not trigger theprocessor to switch on the waveguides. If the lights or fan work without anydisplay or action of program being triggered to start then it can be a possiblefault on the main board or the processor. Sometimes insect secretion can shortthe board and display panel and so the processor can work in such an erraticmanner. If so clean, solder and reset the board. Now identify the parts byclicking this link: http://www.espares.co.uk/advice/microwave/a/5/141/identifying-microwave-parts.html
If there is no heatingplease do not switch on unless you have confirmed the fault. It is possiblethat the Magnetron is faulty, the temperature cut off is faulty or the maincapacitor is short. It is also quite possible that insect can get into the ovenand create soft spots. When you switch on these will create a short and makethe voltage jump and in the long run damage the magnetron. Check this link toknow how a microwave works:http://www.espares.co.uk/advice/microwave/a/5/1443/how-a-microwave-oven-works.html
So Remove cover after disconnecting , check for dirt/moisture inthe cabin, if so wipe clean and dry, check the magnetron and area clean and usean jet air to clean off. Check the wave guide covers, if food had deposited
then it must be checked or replaced. See link: http://www.espares.co.uk/advice/microwave/a/5/1146/how-to-replace-a-microwave-guide-cover.html
Now keeping clear of the microwave, switch on; use a cup of waterinside to test.
If you notice any sparks shut off as the magnetron or the main HT capacitor canbe faulty. replace with same type.
If there is heating without any issues, you can use the microwave safely