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Unplug it from the mains. Call Apple, regardless whether it's a new (still under warranty) or an old one.
 
You probably just blew a capacitor, which is a career-ending injury for the ATV...unless you are VERY handy with a soldering-iron.
 
Keep in mind smoking consumer-grade electronics are baaaaaaaad publicity.

Say the right things in a kind voice and you might get more than a replacement AppleTV. ;)
 
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