That happened because your Apple TV saw a ghost of itself on your network and renamed itself to prevent using the ghost's name. To fix it, turn your Apple TV off and restart your router, then turn the Apple TV back on again.
Thanks for the tip but it didn't work What did work was going into Apple TV settings and changing the name from Apple TV to one of the other options like Apple TV Living Room then changing it back
I've been getting this behavior a lot more frequently for some reason - I haven't touched the network or device configurations. Go figure.
Note, I've seen Macs permanently change their network name due to this - so you need to change it back to what you want then reboot the network to make the change work.
The thing that usually causes Macs to do this is Wake on LAN. With 10.6, Apple changed it to support wireless networks causing this ghost device problem. You can uncheck "Wake on LAN" in the Energy Saver preference pane and that should stop it from happening to Macs.