I have had similar situations when I have 1) changed computers and 2) changed OS from Windows to Apple. My tracks were not deleted I just had to locate them by browsing to their stored location. I keep all of my iTunes on an external hard drive (which I back up to another external hard drive) to alleviate situation 1. I have found for situation 2 the Windows OS could not handle special characters that the Apple OS has no issues with. Windows therefore changed the file name to something with an "_" which is not how the file is recognized in the Apple OS (e.g., if the song name has an !, :, (, ) ...). Once I've browsed to the file to re-establish the link, I've had no further issues. I hope this helps. I haven't found an easier solution to date and it is a real PIA if you have many albums to restore.