iTunes Redownload Music
A while back (maybe two years ago), I ran into the same situation.  I had to e-mail apple support before I could re-download it.  They warned me, saying that I should make backups, and that next time I would have to purchase the music again.  As it turns out, I actually did lose the music a second time (because I was an idiot), and sure enough they wouldn't let me re-download it.

Perhaps their policy has changed now that cloud computing was so prevalent, but it may be worth a try e-mailing them.