How to fix my MacBook Pro that goes straight into sleep/hibernate?

Question

I have a MacBook Pro (mid-2009) that I just installed Mountain Lion on. After the install everything appeared ok, but a day later it goes straight into sleep/hibernate whenever I turn it on.

This happens even when I press Command+R during startup - before it gets to the menu screen it hibernates.

What can I try to fix this?

Answer 1

Usually a SMC Reset is the first thing to do in this kind of situations. As it manages Fans, Lights and Power. To do so, being your MBP an Unibody, if I'm not mistaken, just:

1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift+Control+Option and the power button at the same time.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.

Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.

Answer 2

Before doing an SMC Reset as suggested by @Thecafremo, I decided to clear the PRAM/NVRAM (because the SMC Reset page says: "an SMC reset should only be attempted after all other standard troubleshooting has been performed")

This seems to have been enough to bring the MacBook back to life at least for the moment.

Clearing the PRAM/NVRAM is achieved by holding down Command+Option+P+R during boot before the gray screen appears.