There is no power button on the Apple TV, however rebooting the device is simple. If the Apple TV menu is visible, the Apple TV can be restarted using the following instructions:
On the Apple TV, navigate to Settings -> General -> Restart.
If the Apple TV is unresponsive or does not properly display video, then unplug the main power cord from the back of the Apple TV. Wait a few seconds and plug the power cable back in.
The Apple TV should restart. If the Apple TV fails to boot, you may need to restore using iTunes.
Remember: Never unplug an Apple TV while software updates are in progress!