Try resetting the SMC and PRAM
Laptops with a battery you can remove
Shut down the computer.
Disconnect the MagSafe power adapter from the computer, if it's connected.
Remove the battery.
Press and hold the power button for 5 seconds.
Release the power button.
Reconnect the battery and MagSafe power adapter.
Press the power button to turn on the computer.
Resetting NVRAM / PRAM
Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.