Yesterday the Apple TV got an over-the-air update that broke the awesome PlexConnect app that brought Plex to unjailbroken Apple TVs. If you've already updated, there's a way to fix it.
Unfortunately, Apple's update broke the PlexConnect hack that let you install Plex using Apple's Trailers app. Fixing it takes a bit of work (and you'll need to be familiar with the Terminal), but it isn't anything too hard -- all you need is a microUSB cable and a Mac with Apple Configurator installed. Check out the link below for the full instructions.