Sorry, but officially Apple allows only a single download per purchase. So unless you have a backup of your purchased items (highly recommend and even suggested by iTunes itself) or you have them on an iPod, you may be out of luck and will have to repurchase. You can, however, contact the iTunes Store customer service group through the form at the bottom of this web page:
http://www.apple.com/support/itunes/store/lostmusic/
and explain what happened. Reportedly Apple has in such circumstances authorized a second download. But don't count on it.
If you do have the tracks on an iPod, you can transfer them back into iTunes using the "Transfer Purchases from iPod" command under the File menu. You'll need to prevent iTunes from synchronizing to your iPod automatically or you'll erase the tracks from the iPod as well.
Good luck.
