Go to your home > Music > iTunes >iTunes music. This is where your songs are stored. It sounds like your libary file has become messed up. Simply drag your music folder back in to the iTunes window.
If your music files aren't in your iTunes Music folder then you have either moved or deleted them. You can Spotlight one of the track names to find where you have moved the Music folder to, or if they are deleted you will need to put them back from your backup.
Empty/corrupt library after upgrade/crash
Hopefully it's not been too long since you last upgraded iTunes, in fact if you get an empty/incomplete library immediately after upgrading then with the following steps you shouldn't lose a thing or need to do any further housekeeping. In the Previous iTunes Libraries folder should be a number of dated iTunes Library files. Take the most recent of these and copy it into the iTunes folder. Rename iTunes Library.itl as iTunes Library (Corrupt).itl and then rename the restored file as iTunes Library.itl. Start iTunes. Should all be good, bar any recent additions to or deletions from your library.
See iTunes Folder Watch for a tool to catch up with any changes since the backup file was created.
When you get it all working make a backup!