-plugging the ipod in to a different usb port
-using a different usb/ipod cable (if the above did not work)
-plugging the ipod into a different computer
-plugging the ipod into a different computer with a different cable (if the above did not work)
are both the computer and usb port working fine? plug another device in to the usb port such as a memory stick and see if you have issues.
is the ipod working fine? have you noticed issues before this?
are any of the connectors damaged? (ipod bottom, usb port, usb/ipod cable ends)
does the wire appear damaged? wrapping the wire too tight/kinking can cause a break.