If your Mac display is dark
If your display is dark, check the brightness level, check that your Mac and display are turned on and not in sleep, and make sure cables are connected.
Check the brightness level: Try adjusting your display’s brightness.
Check that your Mac is not in sleep: Press the Space bar or any other key to see if the Mac wakes.
Check that your Mac and display are turned on: If you’re not sure if your Mac is turned on and functioning normally, try restarting your Mac.
Check cables: Make sure that the cables that run between your Mac and the display are securely connected.
Verify that your Mac can support the type and number of connected displays: To check if your Mac is capable of supporting the type and number of displays connected to it, go to the Apple Support Tech Specs website, search for and choose your computer model, then scroll to Graphics and Video Support.
Note: You must be connected to the Internet to do this.
If none of these solutions work: If you have an Apple display and can’t correct the problem, contact Apple or an Apple Authorized Service Provider for service. Repairs performed by untrained individuals or using non-genuine Apple parts may affect the safety and functionality of the device.