This weekend my Nexus 7 went “dark” on me and wouldn’t come back. I assumed the battery had died and plugged it in – no joy. I tried holding down various buttons for 10-15 seconds, etc. – nada.
This post has the solution: hold down the power button for 30 seconds. That did the trick.
Unrelated: the Nexus 7 screen is showing pretty annoying scratch damage. I’ve treated it like I have my iPads and the iPad screens are all pristine. You may want to be a little more careful with the Nexus 7 screen.