If the Mac is working but its monitor says No Signal, select the monitor input that matches your cable and reconnect the cable at both ends. If you use a dock, plug the display directly into the Mac for the test. Open System Settings → Displays, hold Option and click Detect Displays.
01Start with the monitor and a direct cable
Section linkCheck the monitor’s power and open its own input menu. A cable plugged into HDMI needs the matching HDMI input. If you use USB-C, check that the cable and the monitor’s port support video. If a direct connection works, reconnect the dock by itself to see whether the problem returns.
02Try with the laptop lid open
Section linkOn a Mac laptop with Apple silicon, unlock it and choose Allow if it asks whether the display can connect. Leave the lid open for this test. For closed-lid use, Apple says the Mac needs power and an external keyboard and mouse.
03Check whether Displays lists the monitor
Section linkIf the monitor is missing, disconnect it from power and from the Mac, reconnect it and try Detect Displays again. If it appears in Displays but stays dark, check brightness using the monitor’s controls. If the problem began after you increased the refresh rate or resolution, try a mode supported by the monitor, cable and Mac.
04If the problem only affects one setup
Section linkTry just the problem display with a compatible cable. If that works and the display goes dark after you add another monitor, look up the external display limits for your exact Mac model. If a direct connection still fails, try a compatible cable or a different display to see which part of the setup the problem follows.