All content is tied to the account that downloads it, so to be able to try and re-download it you will need to know what the account id is, and its password (though you should be able to get the password reset if you can't remember it : http://iforgot.apple.com). If you don't know the account id then do you still have any of the items anywhere so that you can try to add them to your computer's iTunes (File > Add To Library) - the account id is embedded in each item that you download, so trying to add an item to your library should result in a prompt to authorise its account on it if it's not already authorised.
Not all item types can be re-downloaded (audiobooks are one-time only downloads), and whether you can re-download music, films and TV shows depends upon what country that you are in, and whether they are still in the store. What you can re-download will show under the Purchased link under Quicklinks on the right-hand side of the iTunes store home page on your computer's iTunes. But note that re-downloading an account's items may tie your computer to that account for 90 days : http://support.apple.com/kb/HT4627