iPad won't turn on after a complete battery drain
Assuming it is now charged…  

Hold Home &amp; Power buttons for at least 45 seconds, see if it starts up normally.

If not…


Plug your iPad's USB cable into a computer with iTunes. 
Hold down the Home button [keep holding] and connect to your iPad.
Keep holding the Home button until you see a screen telling you to connect to iTunes.
iTunes should show you a popup saying that there is an iPhone or iPad that is in Recovery Mode and needs to be restored before you can use it. 
Click OK.
Click on Restore iPhone/iPad.


If the above doesn't work, you may need to get a fully operational iPad charger and plug it into the wall. The current and voltage needed to wake an iPad with a deeply discharged battery (or one that's malfunctioning due to age) is higher than a computer can provide over USB.


Plug into a known good iPad type adapter (higher current than iPhone charger)
Wait 15 minutes
Try powering on
Wait 10 minutes
Try a hard reset (hold home button and sleep wake button together for 15 seconds)
Wait 5 minutes
Try powering on


At that point, you can let it charge for another hour and try again or start with a different cable or a different charger. Eventually, you'll need to have the iPad serviced if it cannot wake from sleep due to a very low batter charge condition even when plugged into power.