ipod can't find files

I recently got a new computer, and downloaded most recent version of itunes.

I put all of my songs on an external hard drive.

When I opened "library" in itunes it lists every singler one of my songs from the external hard drive. So far so good . . .

However, it will play none of them. Error message says something about cannot play because cannot find file. Interestingly, I can play the songs by going through back way clicking on external drive, itune, music, songs. So they are there but itunes is not a shortcut for some reason.

Any clue how to make itunes work properly?!?

Thanks