Also depends on what you mean, what the red lights inside are showing, whether it is bad system drive, video card, SMC, whether you can get into Mountain Lion Recovery or not. And whether you hear the chime, it powers on or not.
Unplug everything. No power or USB connected.
Remove all the drives and see if you get the "?" on startup.
If you had made a clone of your system and have that 'sittin on shelf' now is the time to use it.
Hit power button for 10 seconds. Wait a minute. Reconnect bare basics (power, video, kb/mouse)