We are also having the same problem as of last night. First, we wanted to rent a movie and it kept saying the iTunes Store was unavailable. Then the next thing we know, we were locked up on the screen and it seems the remote won't work, even though the lights are flashing on the Apple TV. I tried turning the apple tv off using the remote this morning with zero results so I unplugged it (twice) and it restarts at the same locked screen, then eventually goes to the main menu board, then screen saver, but There is no way to navigate or control it. Maybe this is a software glitch?
I didn't know there was a remote app(thanks!) so I just downloaded it and am going to try using that. Otherwise, I don't know what to do. But at least I know it's not just our problem if others had it happen... on the same day! Must be something Apple needs to tweak. Hope they do it soon!