Second monitor or TV not detected

Steps

1. On the monitor itself, press its Input / Source button and make sure it's set to the connection you're using (HDMI 1, HDMI 2, DisplayPort…). Monitors switch input on their own more often than you'd think.
2. Reseat both ends of the video cable — unplug and plug back in firmly at the computer and at the monitor. Try a different HDMI socket on the monitor if it has one.
3. On the keyboard press Windows + P. A panel slides in on the right — pick Extend (or Duplicate if you want the same picture on both). "PC screen only" is what makes a second screen go dark.
4. Still nothing? Right-click the desktop → Display settings → scroll to Multiple displays → click Detect.
5. Restart the computer with the monitor already switched on and connected.
6. Laptop with a docking station or USB-C hub: unplug the hub, plug it back in, and make sure the hub has its own power if it needs it.

If that didn't work

• Try the monitor with another device (a laptop, a games console) — if it works there, the monitor is fine and the problem is the computer's graphics driver or port; if it doesn't, it's the monitor or cable.
• Try a different cable. Cheap or long HDMI cables fail quietly.