A microwave can be dead for many reasons.
If it goes dead for a while during or after cooking then comes back on, the magnetron is probably overheating and causing the magnetron thermostat to open. Then when it cools, it closes the circuit and allows power through again.
If it's intermittent or random, it may be a bad connection, usually on the control board or a loose fuse holder.
If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.
If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.
If it goes dead or blows the breaker when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.
If the fuse is good, it may be an open thermostat or thermal cutout on or near the magnetron or on top of the cavity / body of the oven.
There should also be a very helpful "mini-manual" hidden inside the unit behind the control panel. It's very helpful when troubleshooting.
Basic disassembly on this is as follows:
1. Unplug it from the outlet.
2. Remove any screws and unlatch any tabs in the vent grille.
3. Pull up and/or out to remove the grille.
4. Remove the screw now revealed at the top of the control panel assembly.
5. Lift up and/or swing down the control panel assembly.
6. Retrieve the aforementioned mini-manual.
Use the mini-manual troubleshooting tips to find out what's wrong.
You can also find helpful exploded view diagrams and order parts by entering your full model number here:
Appliance Parts From PartSelect | Repair Parts For Major Appliances.
GE Microwave 
