ok hold down power button for at least 25 seconds this should reset the tablet
then try to turn on as normal
do the above with the usb lead plugged in to a power source (pc or usb wall outlet) so you know its getting power
if it boots pull the usb plug and see if it stays on
try this and post results
the above works on several allwiner tablets it is supposed to also work if you are stuck in a bootloop
(@ Charlie if you want to try that out next time you get in a loop with build prop cos i dont know if it works on excel not tested)