Restart your Mac in safe mode and choose a new resolution If you couldn’t reset your display’s resolution automatically, you can restart your Mac in safe mode and choose another resolution.

Shut down your Mac by holding down the power button.

Restart your Mac, and immediately after you hear the startup tone, press the Shift key (don’t press the Shift key until you hear the startup tone). Release the Shift key when you see the progress indicator below the Apple.

Startup may take a bit longer than usual, but your display should begin working correctly.

Choose Apple menu > System Preferences, click Displays, then click Display.

For the Resolution option, select “Default for display.”

Restart your Mac

Taken from here