Original file could not be found iTunes?

If this has happened to all your files it because you've moved them (renamed directory containing music) or your iTunes library is corrupt.

If you moved your files you can use something like dreamweaver to do a search and replace on the XML library file and update the worng path to each file with the correct one.

If you have a Mac, you can use tool that automatically fixes this by Spotlighting lost files: http://itunes.pornel.net/