Work through these in order
- Check power and cables. Apple's first step: make sure the display's power cable and video cable are connected securely and correctly at both ends. Check the monitor is on and set to the right input.
- Check the adapter. If you use a USB-C adapter, it needs to support DisplayPort Alt Mode or Thunderbolt to carry video. Charging-only cables and some cheap adapters don't.
- Skip the hub or dock. If the display is connected to a hub or dock, connect it to a port directly on your Mac. If that works, the hub is the problem.
- Reconnect. Disconnect the display from the Mac and from its power, then reconnect it.
- Sleep and wake. Put your Mac to sleep, then wake it. This makes the Mac check its connections again.
- Detect Displays. Choose Apple menu › System Settings, click Displays, press and hold Option to show the Detect Displays button, then click it.
- Allow the accessory. Mac laptops with Apple silicon may ask for permission before a new accessory can connect. Allow it, and check Privacy & Security in System Settings if you dismissed the prompt.
- Check brightness. A display set to minimum brightness can look disconnected.
- Update and restart. Install macOS updates and any firmware update for the display, then restart your Mac.
Lid closed?
If you use a Mac laptop with the lid closed, Apple's steps add: make sure the Mac is connected to power and using an external keyboard and mouse.
Too many displays?
Every Mac has a limit on how many external displays it can drive at once, which also depends on each display's resolution and refresh rate. If one of several displays stays dark, check your model's Tech Specs under display support.
Quick reference
| Symptom | Try first |
|---|---|
| Monitor says "No signal" | Right input selected; cable that supports video; direct port |
| Works directly, not through the dock | Update the dock's firmware; check it supports your display |
| Not listed in Displays settings | Hold Option and click Detect Displays |
| Dark only with the lid closed | Connect power and an external keyboard and mouse |
| Third display stays dark | Check how many displays your Mac supports |