Hello, Unplug the unit from AC power for 30 minutes, and disconnect all of the speakers and other associated equipment. Upon plugging back in, press and hold the power button at the same time for 5-10 seconds to force a hard reset.
If the unit starts up normally start connecting the speakers one at a time (shutting the power down before connecting each one), until one of the speakers causes the unit to shut down. This speaker, or the connection to the receiver is faulty and would need to be replaced.
If the unit will not power up with NO speakers connected, then it needs the output transistors replaced inside the unit. They are not user-replaceable as they are PCB mounted, and thermally coupled to a heatsink. In this case, the unit is in need of service.
To replace the output transistors, you'd typically be looking at between $150-$200 for parts and labor included. I can send you a link to locate a service center if you'd like.
If you need further assistance let me know.
It's almost impossible to tell. A speaker/ wiring problem would cause the outputs to fail, but so can age. I've seen Denon's last for 10 years and have the outputs fail under normal use, and I've also seen them fail after 1-2 years.