I had the same issue the morning after a Fire OS upgrade occurred. It still worked after that evening after the upgrade, but wouldn't even turn on the next morning. I replaced the battery with a fully charged one, but it still would not turn on. I tried all the soft and hard resets, but the tablet is still bricked. I'm going to disassemble it and short the CLK pin to ground to see if I can force a reboot and get it working.