iPod and iTunes go hand in hand like peanut butter and jelly, but it is a problem if iTunes suddenly doesn't recognize your iPod. It could be an issue with the iPod device, the iTunes software or both.
USB Connection Drivers
According to Apple, one of the main reasons iTunes may not be seeing your iPod is that the USB connection drivers may be corrupted. The resolution to this issue is to reinstall your USB drivers for your particular computer.
Apple also suggests that you attempt to reset your iPod. This can be accomplished via the soft reset function. Hold the center button and the menu button down simultaneously until you see the Apple logo come up. Then retry the connection.
Also, attempt to reinstall iTunes. Your version of iTunes may be out of date or corrupted, and it may require a new installation. After installing the new iTunes software from www.apple.com, restart your computer and try again.
If this still doesn't work, check the iPod screen to see if there is a sad face icon. If so, there is usually a hardware issue that needs to be addressed before the device will work again properly.
Check the USB connections and make sure they are secure. It is possible that the USB connector you are using is faulty and may need to be replaced. You can contact Apple to order a new one, or purchase one from one of many authorized resellers locally.