There is a weird issue I've noticed sometimes where the Nexus 7 won't come out of standby/wake up (it IS running and working fine otherwise with music playing, receiving notifications, USB debugging is fine etc, just that the screen doesn't turn on). You can sit there pressing the power button, but the screen never comes on.
When you hold the power button for 15 seconds or so, it performs a hard reset, which forces an instant shutdown and it reboots (giving the appearance that it was off).
As for how to fix it, I don't know. It used to happen to me constantly on stock 4.2.1. I flashed a nightly CM10.1 and the problem went away, so it might just be a bug and not a hardware issue. But I wouldn't consider rooting and flashing a custom ROM a "fix" per se. So I guess we will have to wait for Google to release an update.