macOS Sierra: If you changed your display’s resolution and now there’s no picture
If you changed your display resolution and now there’s no picture, 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 the Mac resets the display resolution on its own.
If that doesn’t work, press the Esc (Escape) key, which should revert your display to the previously selected resolution.
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 for 6 seconds.
After your Mac shuts down, wait 10 seconds, then press the power button.
Immediately after your Mac starts (some Mac computers play a startup sound), press and hold the Shift key.
Release the Shift key when you see the gray Apple logo and progress indicator.
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.