MacBook will not turn on I have a MacBook pro and it won't turn on. Basically I press the power button and the White light at the front turns on. Then a few seconds later it just crashes and the White light goes off then the laptop turns back on but just seems to be stuck you can hear it and the White light stays on (not flashing like in sleep) and that's it. It just stays like that no screen or keyboard lights, nor the Apple logo. I have tried restarting it and tried holding all the key combinations that the apple site suggest, but still it doesn't work. I've tried it with the power cable plugged in and without. 
It's a 2008 model 15". 

I spent ages saving up for the laptop, and if you've eve had a problem with your mac you should know how worrying it is. So any help no matter how big all small would be very welcome. And I'll add a bounty for all my reputation if answers start appearing.
There is a problem with the NVIDIA graphics processors that can prevent the MacBook Pro for powering on. I have a 2008 15" MacBook Pro that had symptoms similar to what you are describing about a month ago. I took it to the Genius Bar and they determined that it was the NVIDIA chip causing it and replaced the logic board for free despite the fact that it was not under warranty and I didn't have AppleCare.

My recommendation is to take it in to the Genius Bar and have them check it out. I have found they have great support and have fixed problems out of warranty (such as this one) on more than one occasion. However, it looks like from the comments on another answer that you have done this already. It might be worth a shot trying again and take the support article with you to see if they change the diagnosis.

Or perhaps this answer might help someone else experiencing the same problem.