From what I know about microwave ovens, I would say that the magnetron (microwave producing tube) isn't working. There is an internal fuse for this, along with the magnetron itself, high voltage diode, capacitor and transformer in the microwave producing circuitry, any of these could cause your problem. Also, there are special switches on the door latch that prevent the production of microwave energy when the door isn't closed properly. These switches are often jarred out of alignment and don't close properly when the door is closed.
If you don't know what you are doing these are DANGEROUS to work on with the covers removed. In my area we have an independent microwave repair shop that will troubleshoot your problem for about $50, which can apply to the repair charge, if you so like.