Apply a charging profile consistent with the battery datasheet. Varta CP1654.

If you lack a battery charger, a low voltage supply through a current limiting resistor can work. Limit charging to 1C rate (<60mA) and increase voltage periodically as battery voltage recovers.

If the charge voltage does not raise above a 1V in a few minutes, the cell is most likely damaged beyond repair.

DO NOT APPLY VOLTAGE OR CURRENTS GREATER THAN THAT SPECIFIED IN THE DATASHEET. THIS MAY RESULT IN DAMAGE TO THE CELL, ELECTRONICS, OR PERSONAL INJURY!

If you do not know how to properly recover the battery, it is recommended that you stop charging at ~3.5V, and allow the onboard charger to complete the rest after re-assembly.