Quoting from MacBook: shuts down intermittently:
This seems to be the only reliable way of starting up the darned MacBook these days. And especially annoying since it's out of warranty and I've already replaced the logic board once, for some Bluetooth issues.
[..] turn the computer on the by holding the power button down. Keep the button down until you hear a single long tone. Once you hear the tone you can release the button, install the recommended updates, and restart as you would expect.
This seems to be the only reliable way of starting up the darned MacBook these days. And especially annoying since it's out of warranty and I've already replaced the logic board once, for some Bluetooth issues.