If there’s no picture on your Mac after you change the resolution
If you changed your display resolution and now there’s no picture on the screen of your Mac, you may have selected an unsupported screen resolution, causing the screen to go black temporarily. The display may reset its resolution automatically, or you may need to take some simple steps to correct it.
Reset the resolution automatically
Wait 15 seconds to see if your Mac resets the display resolution on its own.
If the display resolution doesn’t reset on its own, press Esc (Escape) to revert your display to the previously selected resolution.
Reset the resolution in safe mode
If you can’t reset your display’s resolution automatically, you can restart your Mac in safe mode and choose another resolution.
Startup may take a bit longer than usual, but your display should begin working correctly.
After your Mac starts up, choose Apple menu > System Settings, then click Displays in the sidebar. (You may need to scroll down.)
Click Default on the right to use the best resolution for your display.