I press time to cook and timer runs very fast and dings.

read your manual to get it out of demo mode or online search using make and MODEL

Possibly a bad door switch behind the holes where the latch hooks go into. Usually with the bad switch the timer counts down but nothing runs. It all depends on which switch is bad. Usually 3 switches inside.

You have the unit in demo mode, Follow instructions in manual to exit demo.

It sounds like the magnetron may have either failed or is not being supplied with the extremely high voltage required to run it.

YOUR SAFETY IS PARAMOUNT

This is a job for a professional but if you are safety clued up, here's instruction.
Make sure the !!!!any capacitors are discharged!!!!! before attempting any sort of repair.
Check the door interlock switches first then the high voltage diode with either an AVO model 8 or moving coil meter on high resistance range for short circuit, (DVM's won't show the fault!), the capacitor can fail and go short circuit, the feed fuse on the primary of the high voltage transformer and then finally, the magnetron is best checked by substitution.
Hope that helps.

try unplugging it for about 15 minutes. if that doesnt work, you might as well get a new one. i fix everything myself for just the cost of parts and i have found that what parts cost for a microwaves, its cheaper just to buy a new one.

Here is where you can download the manual
http://www.retrevo.com/search/v2/jsp/mytrevo/myTrevo.jsp?page=man

Yes the problem can be undone also.

The fact that the microwave and convection both do not heat indicated that the oven is in demo mode, check the user manual, otherwise it will possibly be a relay not triggering, may need a pcb replacement, check Demo mode 1st....Phil

Pressing "timer/clock" button 4 times turns on and off demo mode.

I had the same problem and found that the "demo mode" had been enabled. Turned it off and everything worked fine.