It means that iTunes can no longer find the file for the song and is therefore unable to play it. To get them all back into your Library, you need to find the file for each of the missing songs.
Basically, iTunes is a database, the music itself is not "in iTunes". Each song listed in your iTunes Library is a reference to a file for that song and those files are located on your computer and can be seen in Windows Explorer. Have you been deleting, renaming or moving music files in Windows Explorer, or the path to those files? If so, you need to restore each file to its original place, or name.
Is there a common factor with the missing music? For example, was it all added to your Library at the same time, or from the same source?