My MacBook Pro won't turn on. What's wrong?
Depends upon what you mean by it not turning on, there are various causes.
Is the power button pressed and nothing happens? Disconnect all other devices, then a SMC and NVRAM reset.
Is there a white screen and nearly nothing? A hosed EFI firmware.
Is there a grey screen and a loading bar that gets stuck? Either a failed firmware update or corrupted drive/OS X.
Is there a grey screen with a spinning wheel? Bad third party or Apple at boot kext file.
Are you loading but not able to log in? Possible corrupt third party software designed to startup upon login.