itunes original file cannot be found

Why cannot i play my library? It appears with the message "original file can not be found"?
Basically, when I try play my music from iTunes I get a message saying "the original file can not be found. Do you want to locate? When I try to locate all my music is all there in the folder, so why cannot they find. But a little exclamation mark appears next to the song and I can not play. How do I solve this without having to go through my entire library to find each song individually.
This means that in a different place than the first time that I saved it to Tunes. For example, do you have on your desktop and then perhaps moved to the music?
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