Apple TV will not start -

Have you tried to restart your Apple TV ?

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If you can't restart your Apple TV because it's unresponsive, try the following:

• Hold down Menu and Down on the Apple Remote for six seconds, and then release when you see the indicator light on the Apple TV blink rapidly.
• Unplug the Apple TV power cord for three seconds, and then plug it back in.

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Also see this Apple article:

http://support.apple.com/kb/HT3180

See if you can get it running again and let us know.

Then you need to go a step further.

You will need a micro-USB cable.

If your Apple TV still does not respond or if you were unable to follow the above steps

This restore process requires a micro-USB cable (sold separately), a computer with iTunes, and a high-speed Internet connection. This will install the latest software and revert the device to factory settings.

1. Disconnect the power and HDMI cables from the Apple TV.
2. Verify that you are using the latest version of iTunes:
   • Mac: Choose iTunes > Check for Updates
   • Windows: Choose Help > Check for Updates
3. Open iTunes on your computer
4. Connect your Apple TV to a computer:
   • Apple TV (2nd generation): Connect one end of the micro-USB cable to the back of your Apple TV, and the other end to one of your computer's USB ports.
   • Apple TV (3rd generation): Connect one end of the micro-USB cable to the back of your Apple TV, and the other end to one of your computer's USB ports. Connect the power cable from your Apple TV to a power outlet.
5. Click Restore Apple TV from the Apple TV summary page. This step may take some time, depending on the speed of your Internet connection.
   

Also see this Apple article:

http://support.apple.com/kb/HT4367

You're welcome.

Some cameras have a micro-USB cable.

One of my wife's cameras has one so I don't need to buy another.

so far so good

If your Apple TV fails to restore through iTunes……..

• Check that you have correctly followed the instructions for your model of Apple TV.
• If not already, try using a powered USB port.
• Repeat the process but this time try connecting the cable while iTunes is closed.
• Repeat the process with another cable.
• Try restoring the Apple TV on another computer in another location.

Have done all that. ATV connected to my iMac and updated and verified. So that part is OK. When i connect the power cable absolutely nothing happens. Have tried to pres menu+down for 6 seconds, but that trick didn't work either. What can I do???

Try a different cable. Make sure it is capable of data transfer (not just for charging), and if it was bundled with another device it may only be designed to work with that.