Worse case, if it is not coming out of sleep. Remove the AC and battery so it looses power.
Then reinsert the battery and ac and power it up. Does it boot then?
Ok when everything is unplugged AC and battery
Press and hold the power button for 10 seconds so it resets the SMC. When thats done just connect the AC and try and boot does that help?
What model? Do you have the 8600M GPU?
They are known to go faulty, big mess up on NVIDIA's behalf. Sounds like it has gone bust. Apple extended the warranty to 3 years on that part.