At a guess, either:
- you are managing your iPod by using Sync selected playlists, artists, albums and genres and you have changed the selection of music to be synchronised, or
- you have synchronised the iPod to a different iTunes Library
iPods can only be synchronised with one library; If you try to sync with another iTunes Library, you would see a message telling you that all existing music would be removed from the iPod if you Sync with this iTunes.
To remedy to fix it depends on what you did to cause it. If you let us know we can advise you further.