Replying to Itunes Trouble.

Okay this is probably a long shot. But so the other day my brother decides to clear the trash.. So now every time I go to click on a song it says "The song "( blank blank)" could not be used because the original file could not be found. Would you like to locate it? Cancel  Locate"

Sooo uh. Anyone know what I should do. I have a Mac. Oh and it does it for every song. I've tried "locating" it but can't find it. Help please. +k thanks