Did you do everything here:
New cable and different USB port?
Runs this and see if the results help with determine the cause
Try on another computer to help determine if computer or iPod problem
Make sure you have iTunes 11. Turn on the Sidebar. Go to iTunes>View and click on Show Sidebar. You can also do a Crtl+S to show the sidebar. The sidebar is where Devices appears. and Control+B to show the Menu bar