There are many different reasons that a computer may not recognize an iPod. The most common is a completely depleted battery. Another reason is because there is a firmware issue with the iPod itself. Most of the time, there are simple solutions to getting your computer to recognize your iPod again.

Depleted Battery

The most common reason an iPod isn't recognized by a computer is because its battery is completely depleted. The solution to this issue is to charge your iPod and attempt to reconnect after your battery has charged.

Malfunctioning Firmware

Another reason is that there has been a malfunction in the firmware of your device. The solution to this issue is to attempt a soft restore. To accomplish this, depress the center and menu buttons simultaneously until the Apple logo appears. Then attempt to reconnect.

Device Manager Malfunction

Your computer may have lost the ability to recognize your iPod if you are running Windows XP and you have recently installed a service pack. If this is the case, you may need to manually add the device back into your device manager. In order to accomplish this, simply click on "Start" and then "Run." Type in "devmgmt.msc" while your iPod is attached via USB and click "OK." The device should reappear under "My Computer."

USB Cable Malfunction

The USB cable may be malfunctioning. If this is the case, you will need to replace the USB cable. Replacement cables may be purchased at any retail establishment that is an authorized reseller of Apple products.

System Requirements

Your computer may be preventing the iPod from being recognized. This may be because the operating system may not meet iPod system requirements. If this is the case, you will need to upgrade your PC or use another computer.

Source:

Apple Support