I know this Answer may be a bit delayed but I had exactly the same issue after using my Dewalt 18v lithium ion battery to run Christmas lights. The battery got totally depleted and when trying to charge it all that would happen is one flash of the charger then absolutely nothing. No indication of life while pressing the indicator button on the battery either. Apparently these batteries can go into sleep mode if totally depleted. [br]
To cure the issue I got a fully charged identical battery and connect wires to the outer + and - slots. I then connected the dead battery to the other end of the wires in the same slots…..make sure + goes to + and - goes to - . I found I only had to do this for a few seconds then Disconnected all wires. On pressing the indicator button on the dead battery I then got a faint flash…..next I place the battery in the charger and it started to charge as normal and after an hr it was fully charged and worked as normal.