I had the same problem after not using my N800 for 1 year.

You can connect a external regulated power supply to the battery directly. It will wake up your battery again.

I used 3,7 Volt and 100mA current limit. After ~5 minutes I put the battery back into N800 and used Nokia charger.

If your battery gets hot disconnect immediately! You should not connect it more then some minutes - it's not a perfect charger

edit: ok this seems not to be your problem, since it was not charging even if the device was powered on.