It may be a shorted high voltage capacitor or a shorted high voltage diode.
Here are some links you or someone you know can use (in order given) for test help, but read the safety warnings first:
How to Test The High Voltage Capacitor Used In Microwave Ovens
How to Test The High-Voltage Diode (Rectifier) Used In Microwave Ovens (if it is NOT shorted, it's probably fine)
You can usually find helpful exploded view diagrams and order parts by entering your full model number
here.
There should be a "mini-manual" hidden inside the unit behind the control panel or hidden on the left side behind the grille, which is very helpful when troubleshooting & testing.
If you still have trouble, feel free to let me know.