Try to unplug for five minutes, be patient and wait five minutes, plug back in wait one minute power on if unit powers on and stays on press and hold ENTER and VIDEO 2 at the same time to try a forced reset.
If that fails to resolve your issue try this , unplug for five minutes again , plug in and wait one minute , power on , press and hold "down arrow" and the "zone2 on/off button" for a few seconds.
Some owners have stated that they have had better luck leaving unit unplugged for up to "Ten" minutes, so keep that in mind if the first attempt fails.