If you think your Nexus is dead, check for loose screws under the ribbon connectors.
tl;dr: Check out whether one of your missing screws has got stuck in the tiny space underneath the ribbon connectors (CHECK ATTACHED PHOTOS)
long story: I'm far from wealthy, but I wanted to get my parents some piece of electronic that they can surf the web and have fun with. I found this cheap used Nexus 7 online and today I've been basically playing around with it all day checking out different roms and stuff.
At one point I had to unplug the battery because I couldn't shut down the tablet. When I did that, while holding the tablet I shook it up a little and heard something loose in it. I noticed there were screws missing. One of the screws was poking out from underneath the charging ribbon. Without taking off the ribbon I tried to wiggle it around a bit with a screwdriver, hoping it would come out, but instead it got stuck inside a little further. I noticed the noise when shaking the device was gone, so I just put on the cover back on and tried to start it. It wouldn't even light up.
I obviously tried everything suggested on the web. Holding the power button for 30s and such. I plugged the Nexus for charging and heard a little buzzing sound coming FROM THE TABLET. I got curious, so I took off the back again, removed the battery and plugged in the USB charger again. The buzz was still there and it was coming from around the charger ribbon. I unplugged it, removed the screw that got stuck inside and the tablet works again. It must've been causing a short circuit, or something.
I'm so ****ing happy, you have no idea.
Thankfully it works flawlessly, the battery is fine and it charges alright.
I really hope this helps somebody out. I thought the tablet was done, but thankfully I'm as lucky as I am dumb.
Feel free to add this suggestion to any Nexus 7 Helper topic of any sorts.