Unplug your Apple TV. You still need to power it back up, but before you do that, make sure to plug a mini USB cable in the back of your Apple TV (right below the HDMI port) and then to a USB port on your computer.
Once your Apple TV is connected, plug it back in. iTunes should open (if it’s not already running) and recognize it. Now you have two options, but since we’re here to fix it, we want to click “Restore Apple TV…”
Click the “Restore and Update” button. Once you do this, the latest version of the Apple TV software will be download and installed, thus restoring it to minty fresh condition.
Once the process is complete, you can safely eject the Apple TV, and it should now boot after you reconnect it to your HDTV.