'''If this is still under warranty I'd take it to a Genius bar.''' There are so many things it could be... If it's out of warranty and you're willing to DIY you could '''follow the guide to open it. Remove the battery '''overnight (8hrs) and see if it boots powered only by the magsafe (don't screw it back together - just reassemble on a flat safe surface before applying power- you'll want to put the battery back eventually). If it boots gracefully shut down, remove the magsafe and reconnect the battery... see if that boots.
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