the Easiest way ( with a ton of songs ) is to remove all the songs from the library. then re add them all.

On Mac ( I assume the same on windows ) it will ask if you are sure you want to remove the song( songs) from the library say YES then a second warning pop up will ask if you want to move the file( files) to the trash say NO.

just select all and you can do it in one shot

then go to the folder that the files are kept and drop it in the itunes window it will re add all the songs

only problem with this is it will lose all your play lists.... but with a ton of song's it may be worth it .