You can see the purchase history on your account via your Mac's iTunes : log into your account on your Mac's iTunes via the Store > View Account menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases.
Depending upon what country that you are in (music can't be re-downloaded in all countries) then try deleting the incomplete tracks from your iTunes library and redownload them via the Purchased link under Quicklinks on the right-hand side of the iTunes store home page on your Mac's iTunes.
If you aren't in a country where you can re-download music then try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com
If the 'report a problem' link doesn't work then you can try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption