Exlamation points next to my songs

I could really use some help. Most of my songs in my library have exclamation points next to them and will not play. It says the original file cannot be found. These songs were all working fine all along and now all of a sudden the file cannot be located? I don't understand what could have happened. I can go back and locate each one individually but that would take forever, there are thousands of songs... Is there an easier way to locate where they come from and fix them, or should I delete the library and import again?