I believe its the mountain lioin update, Ive been on the phone with apple for almost an hour going from tech to tech, tried a few things still a problem, i mentioned that I read a bunch of threads about the problem and they all say that havent heard any.
I think it has to do with the way ML puts the computer to sleep
I've encountered this same issue this morning. First use of my Apple TV 2 since upgrading to Mountain Lion. Mac was awake when I tried (I actually started out by trying the AirPlay mirroring from the Mac, which worked fine). When I tried to access my library in any way, the Apple TV displayed: "There are no [videos/playlist/songs/etc.] in this library."
I have just managed to get mine working by restarting the Apple TV (Settings/General/Restart) and it's playing music as I type. I've been reading reports mentioning ML's sleep, so I guess time will tell whether this restart will be a permanent fix.