why is my ipod not charging?

Since your computer seems to acknowledge that the iPod is there it's probably not a problem with the cable or dock connector. Could it be that the USB port on your computer just doesn't have enough power to get that completely drained battery going again? If you have a power adapter that lets you charge your iPod from the wall current that would be worth a try. If you don't have one you could go to an Apple Store or any store that displays iPod charging in a dock, plug your iPod in there and see if you get the battery charging screen. It could take a few minutes to respond, so give it a chance. Make sure it's in a dock connected to wall current and not connected to a computer USB port and not even one of those speaker system that dock your iPod.

Otherwise, you might want to give us a bit more info on precisely what that error screen says.