Why might an Apple TV stop playing purchased content from iTunes?
The explanation I got from Apple is there is some maintenance being performed on an iTunes server this weekend, which is affecting AppleTV's ability to authorize content.  Even if you are streaming directly from your PC/MAC to the AppleTV, there is still some validation with the iTunes store that occurs.

In the meantime, you can use Airplay to continue to stream content to your AppleTV.  More info on using Airplay can be found here: http://support.apple.com/kb/HT4437.