Unplug device. Hold down power button for 60 seconds and then let go of it. Plug in the power cord. Now hold down the power button a few seconds and it will (should) come on. Worked for my Zenpad 3s 10.
I had been playing game and folded the case cover over and headed into a meeting, battery was @85%...3 hours later I returned to find the tablet dead and not appearing to charge/turn on at all. So I googled for a few minutes and found the 60-sec power button and charge cord & then power button trick. Power shows 68% on the tablet now so it actually never drained....must have just frozen in the game or something???