how do i load past purchases

I have a big problem apple doesn't seem to know how to fix it. I removed my purchased songs from my computer and put them on an external drive. The external drive crashed. I can't redowload my purchases because itunes says they are allready downloaded. How do I convince itunes that the songs are no longer on my computer so i can redownload them. I've tried a number of things with apple but n osuccess. Can anyone help?