Hi. For all those who are suffering from "sudden death" of their Nexus 7 1st gen. I have found possible solution, well at least it worked on mine
So thats the story : my sons nexus run out of battery, switched off and thats where the problems started, when plugged in (wall charger, usb) nothing happened on the screen, no signs of charging. For 1 week I was trying playing with battery cables, HW buttons combinations, different usb cables and chargers, all forums, advices and nothing. Nexus lost his life when battery came to 0%. But today I tried something different. Nexus battery has 6 wires coming out, most important are 2 reds and 2 blacks, I had spare USB cable which i cut and separated both black and red wires. Every cable have more small wires inside of it, so I've made 2 reds ones and 2 black ones from my usb cable and pushed them into battery slot (be sure to put red to red and black to black). I just wanted to see what will happen when battery will be charged straight from charger. Nothing to loose as Nexus was dead anyway. Then i plugged that usb cable to genuine wall charger, left it for about 10mins and then fitted battery back into nexus. Voila ! Nexus booted straigh away with 6% battery and now is charging like always. Looks like battery needs to be kicked in the ass with some volts to come back to life.