migmog, oftentimes nicads, NiMH and lithium cells develop dendritic 0Needle-like) crystals internally and short the cell out, hence it appears to be dead.
I technique I have used successfully in the past is to clear (vaporise) those crystals by "flashing" the cell by connecting it VERY briefly across a car battery. Now, care is needed; do not attempt to connect it for more than a fraction of a second. The best way is to hold the negative terminal of the cell and quickly brush the positive terminal of the cell with the positive wire from the car battery.
Pause, and check the cell's voltage; if it has ben restored - success!
If not, wait a few minutes and try again.
This member accepts NO responsibility for any conflagration, death or maiming as a result of your trying this!
Only joking, really.....I hope.
Ron.