Try connecting your charger then holding down the power button for 20 seconds then disconnect the charger and hold the power button for 20 seconds again . then release and try to boot normally. If this cycle brings no joy open the tablet and disconnect the battery. It will involve a soldering iron but if you disconnect for a few minutes it will clear the memory and let you have a clean boot which should allow the tablet to boot. this guide will help
[guide|39918]