A quick PRAM Reset, or Parameter Reset, is an easy way to resolve issues involving various that include but may not be limited to:
- Speaker volume
- Screen resolution
- Startup disk selection
Here's the steps you need to take:
- Shut down the computer.
- 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. You must press this key combination 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.
Additional Information for Third-party displays
Per Apple Support, if you have screen resolution issues with a third-party display (monitor) and resetting the NVRAM/PRAM does not help, try using the display's built-in menu system if it has one. For details, see the manual that came with your monitor. There may be buttons on the front of the display to configure the internal settings and screen geometry. The display may include an "automatic" adjustment mode too.
Tip: If you use a DVI connection and you must click "Detect Displays" (in Display preferences) several times for your third-party display to be recognized, try using the monitor's built in menus to set the DVI connector to a digital signal.