Zap your PRAM
A small amount of your computer’s memory, called “parameter random-access memory” or PRAM (pronounced p-ram), stores certain settings in a location that OS X can access quickly. The particular settings that are stored depend on your type of Mac and the types of devices connected to it. The settings include your designated startup disk, display resolution, speaker volume, and other information.
Power off your computer. Then power the device on while simultaneously holding down the [command]+[option]+[P]+[R] keys. You should hear the Apple chime before the computer reboots. Continue holding the keys down until you hear the second Apple chime to clear the PRAM.
Resetting PRAM may change some system settings and preferences. Use System Preferences to restore your settings.