At nine years old, it's about due for replacement anyway. But if you really want to tackle it, a couple of troubleshooting questions…
1 - Does the turntable turn?
2 - Does it make a buzzing noise or just sit there looking back at you?
The two most common problems are the internal fuse and the door latch. But if the convection oven portion still works, and the turntable still turns, chances are the fuse and door latch are both still working fine.
If you hear a buzzing noise, the most common problems are the power diode, the capacitor, or the magnetron. The capacitor in particular can shock the living daylights out of you, but all three parts are replaceable. I don't know whether you'll save any money replacing any or all of those compared to buying a new machine.
You might consult this webpage for instructions on how to replace a given component…