Hello,
If the lithium-ion battery has dropped below the voltage threshold (below 2.5v per cell), due to having too low a charge from being drained significantly, then the battery becomes unusable.
The onboard circuitry on the battery prevents recharging of the battery when the voltage is below the threshold, because it's dangerous. In a nutshell, when a lithium-ion battery drops below the voltage threshold, the cells can become unstable and electrical shorts can develop, resulting in overheating and potentially fire and risk of explosions.
Onboard circuitry in most modern lithium-ion batteries today however protect against over-discharging by cutting off power before the voltage drops to or below 2.5v.
There are devices out there which, amongst other things, are capable of brining lithium-ion batteries out of the 'sleep state' by charging it with a low current until it reaches the correct voltage threshold
Battery charger and battery analyzer experts - Cadex Electronics Inc. but they are extremely expensive and not intended for consumer use.
My advice, buy a new lithium-ion battery.
