Some items you’ve previously purchased are not eligible for redownload via iTunes in the Cloud and thus, will not appear on your Purchased page. This content includes ringtones, audiobooks, movie rentals, items that have been modified or removed from the store since you purchased them, anything that may have been refunded, and in supported countries, certain movies.
To learn what previously purchased content is available for download in your country, visit this article:
iTunes in the Cloud and iTunes Match Availability
For some movies, you will see the message "When purchased, this movie will not be available from iTunes in the Cloud” if the movie can't be downloaded again. This message appears under the Buy button on the iTunes Store product page.
This is why you should always back things up:
iTunes: Back up your iTunes library by copying to an external hard drive
If you're using a Mac, you can back up your library automatically with Time Machine. This article will show you how:
Mac 101: Time Machine