Help with itunes music?

Did you move the files once you first listened to them in iTunes? Such as downloading them to your desktop and then later moving them into a folder, for example. If so, iTunes doesn't know where to look for them and you can just point it to the new location. If it is the whole batch you had, just delete them all out of iTunes (select all, delete) and reload them in by doing "Add to Library" and pointing to the new location, it'll be easier than doing each one individually.