Zap the Parameter RAM (PRAM). Start holding the keys prior to the grey screen after boot, hold them through the reset and an automatic reboot, through the second startup tone. Resets settings such as: display / video settings such as refresh rate, screen resolution, number of colors, startup volume, speaker volume, recent kernel panic info if any, DVD region setting. Be sure to go into System Preferences afterward and set the clock and timezone, and speaker volume.