I purchased songs from Itunes, downloaded. They play in the songs purchased file and show that they have been downloaded. However they willnot play in Itunes on my computer (Win 7). I click on the individual song get "...original file couldn't be found"

I purchased an album plus an extra song by the same group from Itunes store. They show up in the purchased file. The downloaded button shows they are already downloaded. When I go to my Itunes downloaded purchases the songs are on the list but they wont play. I get the message, "The song " " could not be used because the original file could not be found. Would you like to locate it?" I click on locate but can't fine any of the songs. I can't download the songs again from Itunes. I have Win 7. Can anyone help me please?