I had trouble with my iPod Classic and Windows 7 earlier, this has been resolved now (see General Software forum for my thread).
What Operating System (Mac, Windows XP, Windows Vista, Windows 7) do you use?
Have you downloaded the latest version of iTunes?
How do you connect the iPod to your computer?
As said above, a bit more info would be helpful. The way you asked your questions is pretty much similar to "my car won't run, what is wrong with it"...