After an hour or so of charging (plugged into a wall outlet, NOT a PC, and make sure it's an ipad charger, not a 5 watt phone charger), do a hard reset while the ipad is still plugged in. To do a reset, hold down both the power button and home button until either the device boots up or for 30 seconds if it doesn't boot immediately...booting on a basically dead battery, it can take the device a bit longer to respond to the buttons than usual.
If that doesn't work, charge for another hour & repeat. It might take a few tries.
I had to revive two 1st gen ipads this way a month or two back. It took quite a while to wake them from the dead, but they did both eventually come back to a fully functional state.