Have you tried using a Wall USB Charger or if you are using a Desktop, then use only the USB2.0 port at the back of the PC, as the front USB port does not have enough power to charge iPod batteries.
If you have tried the above, and still no resolution, then bring it to a Ipod Repair clinic to have the battery replaced. Google for "iPod Repairs" and go to the one near your Area.
Have a nice day!