I've only had my Nexus 7 for a couple of weeks and could have cried my eyes out when it suddenly went dead and would not turn on again. Luckily, I sought help from this forum which solved the problem.
Knowing that I had enough battery life before my nexus turned off, I tried several different combinations with the power key, power and volume up, volume down etc..... but nothing worked. I then read the entry about removing the back cover and disconnecting, then reconnecting the battery. I was a little worried about doing this but through pure frustration I took the advice. I popped off the rear case which was easy, I even managed it with just my nails, no tools required. Then I did nothing because I had no idea what was what inside so I popped the case back on, pressed the on / off button again and my nexus turned on as normal as if nothing was wrong. Only data which appears to be missing is my calendar but I'd backed it up via cloud so it can easily be retrieved.
I hope this helps and prevents anyone else from nearly having a heart-attack and melt down as I did.