Is it a Apple TV 3? Is the LED on or does it blink?
If you have already unplugged and rebooted it, you can try this:
Unplug the HDMI cable and the power cable. Connect your ATV to your computer using a micro-USB cable. You can find the micro USB socket on the backside of your ATV. Open iTunes. You will find the ATV on the left side under "Devices". Click on it and choose "Restore". After restoring connect the ATV to your TV.
Hope this helps.