If restarting the computer and iTunes doesn't work you can try one of these suggestions.
1. If items appear (maybe in grey or different from working songs) but don't play you can try to delete them from your library so they will no longer seem to be there to the iTunes store.
a) select the item
b) press option and delete key assuming a mac keyboard
c) yes, send them to trash
d) restart Itunes, go to store, check for available downloads, and select download all
HERE IS ALTERNATE FROM APPLE SUPPORT
Note: Downloading of previously purchased movies is available in the United States only.
- Open iTunes 10.6 or later on your computer. (You can download the latest version of iTunes here.)
- If you're not already signed in, click Sign In, and enter your Apple ID and password.
- After you've signed in, click Purchased on the right side in the iTunes Store under the QUICK LINKS section.
- From your Purchased page, click the tab for the content type you're looking for (Music, Movies, TV Shows, Apps, or Books).
- Click Songs or Albums to change the page view and All or "Not in My Library" to view your purchased content that currently is not downloaded on your computer.
- Click the download icon to the right of each item to download that item.
Notes:
- To download previous purchases on a computer, your computer must be authorized for the Apple ID that was used for the original purchase. For more information about iTunes Store authorization and deauthorization see, this article.
- If they are offered for purchase in 1080p, you can download previously purchased 720p HD TV shows and movies in 1080p. Learn more about downloading in 1080p.
- Previous purchases may be unavailable if they have been refunded or are no longer on the iTunes Store. Downloading previously purchased movies and TV shows requires iTunes 10.6 or later.