Q: part of my library has disappeared

Sounds like your iPod's filesystem may have become corrupted.  As to how, there are several factors, including a damaged drive or glitch in the iPod's software.  On the iPod head to Settings -> About.  Is the Other section unusually large? 

To try and correct this issue, first try resetting the iPod. To do this, press and hold both the Select (Center) and Menu buttons together long enough for the Apple logo to appear.

Otherwise, you'll need to reconnect the iPod to iTunes and restore it to factory settings. This process wipes all content from the device, resets all settings back to their defaults and reloads a fresh copy of the iPod's software onto the device.

Restoring iPod to factory settings

If the content on the iPod is no longer in your iTunes library, you'll want to use this article to walk you through the process of extracting it from the iPod and importing it back into iTunes before proceeding with the restore.

Copying Content from your iPod to your Computer - The Definitive Guide

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