Currently Being ModeratedMay 6, 2013 2:00 AM (in response to aud18)
I don't know what cause the songs to disappear from the iPod. You can easily sync everything back, since all of your songs are in your iTunes library.
To use automatic syncing, you need to set up the iPod's Music tab in iTunes.
Connect the iPod and select it in iTunes. There is a row of "tabs" (starting with Summary).
First, it is possible that some type of data corruption caused the problem on the iPod. Therefore, you may want to do a Restore on the iPod in iTunes. The Restore button is there, on the iPod's Summary tab. This erases the iPod, re-installs its software, and sets it to default settings, giving the iPod a "fresh" start. If the Restore process gives you the option to put your entire music library on the iPod, and that's what you want, you may not need to read the rest... but it's good to know how.
Select the iPod in iTunes again, and this time, click on Music next to Summary. This is the Music tab, where you tell iTunes how to sync the iPod with songs. Sync Music is checkmarked for automatic syncing.
If your entire iTunes music library can fit on the iPod, you can choose to sync Entire music library. That's probably how the iPod was set to sync music before.
Otherwise, you can choose to sync Selected playlists, artists, albums, and genres. Create your selection of songs below. For example, if you want to keep it simple, create a new regular playlist in iTunes. Put ALL songs you want on the iPod on that playlist. Back on the Music tab, find and checkmark that new playlist under Playlists. You can, of course, select any combination of playlists, artists, albums, and/or genres that fits on the iPod.
When you click Apply, your selection of songs on the Music tab syncs to the iPod. Going forward, any library changes that are related to the Music tab settings sync automatically when you connect the iPod, or when you click Sync. So, if there is a playlist with your iPod's songs, you can just update that playlist (add and remove songs) whenever you want; iTunes automatically updates the iPod with the same changes at the next opportunity. If the setting is to sync the entire music library, than ANY change to the music library syncs to the iPod.