iTunes suddenly refused to recognize my iPod Touch when I plugged it in even though it shows up on Windows 8. I was pretty sure it had worked before.
Apple has a troubleshooting guide for just this scenario. http://support.apple.com/kb/TS1538
Under Step 5 (Verify that the Apple Mobile Device USB Driver is installed), I found the iPod under “Portable Devices” and followed the steps to update the device driver. The driver is in the “C:\Program Files\Common Files\Apple\Mobile Device Support\Drivers” folder. Once that was done, I restarted iTunes and my iPod was recognized immediately.