"first remove the battery from the device for a minute, and put it back in.
Leave the device to charge 2-3 hours, and then, still connected to the USB cable, remove the the battery from the device a few seconds and reinsert it. If it boots--leave it to charge to 100% (check Options > Status for the percentage). If it doesn't reboot on the first attempt, leave it several more hours. I've had one charge for two days before it came back to life after an extreme battery depletion. Every 3-4 hours, with the device still connected to the charger, you can remove and replace the battery to see if it reboots."
From J. Sanders
Teostar, the reason we are so concerned about the battery is it has been depleted and left without a charge for so long that its possible it has been damaged. L-ion batterys become damaged with complete discharge cycles, letting an empty battery sit around can damage it beyond repair. L-ion batterys are stored properly with a 75% charge, proper use is recharging before the battery falls below a 50% charge. It is quite possible your battery has been damaged, but it may repair itsself by following Ghostryders suggestion and the battery diagnostic I detailed earlier.
Also, an 8100 battery is less than US $10 with shipping on Amazon, its an easy, cheap fix if nessicary.