Just turn the Apple TV off and back on again, but by physically cutting the power
Oops, sorry I'm an ATV2 owner, so not as much help with the original ATV.
All I'd suggest is after trying the above you might want to restart your computer, or at the very least iTunes. Beyond that, I'm sure that an Apple TV expert on here who can help!
I own both tv & tv 2 and I would suggest going into computers on your tv and add your shared iTunes library again. It should pop the code up on your tv screen and force iTunes to see it.