Maybe this will help someone, but mine wouldn't turn on either. What worked for me is I took out the sim and battery for 10 mins, did the hard reset method by holding down power for 15 seconds, then power+voume up for 15 seconds, volume+up+down for 15 seconds, and volume up+down for 15 seconds. My theory was to remove any remaining electricity in the motherboard. This worked, and battery showed 100% power when booted.
The combination of buttons was probably an excess of combinations, but might as well try all in one go and see what happens. Not once did I have a charger/cable connected except the night prior when I left it charging all night, and woke up to a dead phone.