First check is to be sure that your charger is set for the correct number of cells.

If you have a fail safe type switch, they draw a small amount of power when in the off position and can drain a battery to nothing if left pluged in for a long time.

If everything is set correctly, and you have tested the pack with a loaded voltmeter to be sure that it is low, you can SOMETIMES bring them back by charging for a short time with the charger set for NiCd. ONLY charge this way at a very low rate, in your case about .2 Amp and watch the charger, once you get to 3V per cell, switch to the normal LiIon charge.
Once you get it charged you will want to cycle the pack a couple of times to measure its capacity. If the capacity is not back to full normal capacity, I would replace the pack. Batteries are a lot cheaper than airframes.