Mac OS X 10.6: If your portable computer’s screen suddenly goes black
Your computer may be in sleep. Press any key on the keyboard.
If the screen is still black, your portable computer may be out of power. Plug it into a power outlet and restart it.
To restart your computer:
Plug the power adapter into the computer and into a power outlet.
Unplug any devices that are connected to your computer except the power adapter.
Hold down the Command (⌘) and Control keys while you press the power button to restart the computer.
Let the battery charge to at least 10 percent before plugging in any external devices and resuming your work.
To see how much the battery has recharged, look at the battery status icon in the menu bar.