Itunes can't find my music

Okay so i have been using the website listentoyoutube.com to get my music from youtube and it usually goes straight to my itunes so i can transfer it to my iphone. But about a week ago i upgraded the itunes to the latest software and it won't let me transfer any of my songs as it did before. Next to the unknown songs there's an exclamation point. How do i locate these files?! 1

You have to find that specific file that contains that song though if that file got deleted then that means it no longer exists.