The first thing you should do is see to it your cables are both correct and secure.
confirm you’ve got enough power.
If you’re using a MacBook, try plugging it into power.
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Disconnect and then reconnect your cables.
Restarting the external display may give your Mac a fresh shot at recognizing it.
double-check your Mac supports more than one additional monitor.
Ensure that any adapter is compatible.
If you’re using an adapter, ensure that your monitor and cable combo is compatible with your Mac.
Some Macs use onlyUSB-Cor both Thunderbolt and USB-C connections, and some Thunderbolt variations require a Thunderbolt-specific adapter.
Check resolution prefs by going toSystem prefs>Displays.
Press and hold down theOptionkey.
This will reveal a hiddenDetect Displaysoption at the bottom right.
Click thisbuttonso that your Mac can scan for a connected display.
On older macOS versions, get to this option by selectingSystem Preferences>Displays>Scaled.
Still inResolutionsettings,Ctrl+clicka resolution on the list > and chooseShow Thumbnails.
Select a thumbnail other than the default option to see if this prompts a change.
You may see a radio button next toScaled.
If you do, choose a different resolution from this view.
Deselect automatic brightness adjustments.
UnderBrightness, uncheck the box next to or deselectAutomatically adjust brightnessand manually brighten using the toggle.
If the external display’s brightness is too low to register, this could resolve that issue.
Give It a rest for a few seconds.
Go ahead and power cycle your Mac, but keep the external display connected.
When all else fails, start all the way over.
Repeat the steps for the Display prefs if the monitor still doesn’t work.
When you’re going through this process, be sure to reconnect everythingbeforeyou power your Mac back on.
Increase your chances for success by avoiding daisy-chaining several adapters and cables together.
Macs tend to work best with external displays when an Apple-branded or compatible cable and adapter are directly involved.
Connect the monitor using HDMI, Mini DisplayPort, USB-C, or Thunderbolt ports.
Position the monitors and power on the Mac.
OpenSystem Settingsand chooseExtended displayfrom theUse asdrop-down menu.
Or, selectSystemPreferences>Displays>Arrangementand uncheckMirror Displays.
To set an external display as your main monitor, openSystem PreferencesorSystem controls>Displays>ArrangementorArrange.
One of the displays will have a white bar above it.
Click and drag the bar to another display to set it as your main display.
Or, chooseSystem Preferences>Energy Saver> and move the slider toNever.
Your Mac display won’t turn off now.
