Whats happened then is iTUnes basicly "Copied" its empty library file onto the iPod. When you connect the iPod to iTUnes it copys music over AND the library/database iTUnes has. Id say the music is there, but the iPOd Library hhas no "Link" or "Direction" to ponit to these files so it shows nothing.
The only option you have now is using yamipod
http://www.yamipod.com/main/modules/home/ and seeing if it can recover these songs for you off the ipod. Read the "docs" on yamipods home page here to learn how to use it and what feature you need to use to recover off the iPod (Like Find lost music and repair iPOd)
http://www.yamipod.com/main/modules/docs/
If it can not recover songs off the iPod, and you have no backups made, your going to be VERY limited in your options. you will have to "Re" import all your CDs. All songs you bought online you will have to email apple and see if they will offer a "Re" download of them. Keep in mind there under NO obligation to do so, but most have had success in emailing them and getting an offer to redownload songs they bought and have not be modified on the store. here a link to email the music store if you need to as a last resort
http://www.apple.com/support/itunes/musicstore/download/
To backup music once you recover everything you can go here and follow steps
http://www.apple.com/support/itunes/musicstore/backup/