I had this exact same problem. No advice about thumping the back, nor would it boot to the recovery screen by holding buttons down and all that (3 different variations attempted). I tried disconnecting the battery and still nothing. Mine kept appearing like it was trying to charge, but then kept rebooting, even while plugged into a charger, left overnight and all next day.
What worked was using the OEM charger. I was using an aftermarket charger, but this isn't powerful enough to get the required "umph" when the battery was completely dead. The charger I was using worked fine while the battery had some charge on it, which is why I didn't think this would've caused the problem, but made sense afterwards. Good luck.