What kind of files are you trying to sync?
Have you synced them before? If not, are they compatible with the iPod's supported formats?
While syncing have you noticed at which file the syncing pauses and you get this message? Try to identify the specific file and check if it exists and is playable in your library.
Try managing manually your library as shown here: http://support.apple.com/kb/ht1535
Remember iTunes library is just a list however the actual content is stored in another location in your computer. Have you deleted anything recently?
You can check the details about the media folder were your music is stored in the following article: http://support.apple.com/kb/HT1364
Details on what are the library files and how to restore your library in the following articles:
Here is what i found about this. Straight forward and not a big fix needed. The message ' the required file cannot be found' is referring to the song that it is trying to transfer to the iPod. Try shutting iTunes down and reopen it and sync it again.
1-Try and transfer again and look in the iTunes window at the song that is trying to transfer. Then go into the main library and uncheck that song. Then continue to update the iPod. Do this each time you get that error message when syncing the iPod.
2-Make sure the song that is trying to transfer to the iPod is in the iTunes folder. Check and make sure.
3-If it is then when done syncing the iPod re-import the song(s) into iTunes.
When that message appears when syncing the iPod the problem is the song(s) & iTunes cannot locate it/them to transfer to the iPod.
Good luck and let us know when you are jamming again :)