Try this - Hold power button in for 10 seconds, or until keyboard light goes out, then release and press power button. If it powers up, you have encountered the well reported sleep/wake issue on MBAs. Look at the following threads:
If it doesn't power up, then you have a dead MBA and the AppleStore is your next destination!
One more thought if the above is not working for you -
I had something similar but using the 10 second power button press didn't fix it - the machine came back on, but the screen was still black. Apple care thought it was a dead video card and I wasn't happy !
However at the genius bar they did a P Ram reset and it was all fixed.
Here is how you do it: http://support.apple.com/kb/ph11243
According to the genious bar, the problem was that a duff app had fixed the screen brightness to 0 in memory that persists when the power is off. Pretty scary, but a simple fix.
Sadly I also have the sleep/wake crash issue.. and that has not gone away. Gerrr.
Apple have no understanding of the problem, no root cause and, therefore, no fix.
Take it back and get a refund, Apple may never resolve this two year old issue.
Best fix, if you still want an MBA, is to buy a 2010 version (about 35% of the new cost) and use the money to repair it if it ever goes wrong.
Meanwhile, join us on Fac ebook, search for - MacBook Air screen goes black - and help us warn others of Apple's poor build quality and frequent failures (the Australian government forced Apple to provide a two year warranty on all products as they're so prone to failure, they haven't needed to do that to any other computer manufacturer...