How does it sound when you calibrate the speakers (page 75)? THAT is how the basic hardware is verified. Anything that doesn't work after that is usually a setup or source issue. Read the manual.
Turn the volume control to something medium.
The errant switch, control or condition may change and you will suddenly release the amp's full power and possibly destroying your speakers. If a signal isn't audible at 1/2 volume it's probably not there.
Carefully examine the front panel for clues like a misplaced Source Selection, Mute, Tape Monitor control or Multichannel Analog Input selected.
There is a good chance that a common control may have developed a high-resistance or 'dead' spot through defect or idleness and is causing your symptom. Turn the POWER OFF and operate every control throughout its range a number of times, especially rarely-used ones like Tape Monitors and the Mute control.
Turn the volume to something reasonable and see if that helped.