Go to the iTunes Store, log into your account, and click the Purchases link under the Quick Links. From there you should be able to re-download some or all of your purchased content. Note that not all content has been licensed for re-downloading in all countries at this time. You can see what content you can download here:
You can also re-download content using an iOS device.
For full instructions, see:
If a track for which the file is missing is still listed in iTunes, you'll need to delete the listing (just highlight the item and press "delete") before the download icon will activate.