Turn it off. Plug in the charger. Let it sit, charging, for 24 hours. The first 5 or 6 hours that a dead battery sits on the charger it gets almost no charge - it's not a linear thing, 10 hours to charge, so 2.5 hours is 1/4 charge? No. 2.5 hours is like now it's first going to start charging. (Strange electrochemical reaction taking place in there.)
If after 24 hours of charge, it still doesn't power up, try a hard reset.