1. Press the control key, the command key, and the power button simultaneously and hold for three seconds to try to restart the computer.
2. Reset the PRAM. After pressing the power button, hold down these keys simultaneously: Command-Option-P-R, until you hear the startup chime at least one additional time after the initial startup chime.
3. Reset the PMU for the MacBook And MacBook Pro by removing the AC power, removing the battery and then holding down the power button for five seconds.
4. If you recently installed additional memory, make sure it is correctly installed and is compatible with your computer. Remove the new memory to see if the computer starts up properly without it.