Second monitor not detected
Swap the cable before you touch any settings — cables are the single most common cause and the cheapest to rule out. After that it's the port type, then the display's own input selection.
Swap the cable
Do this first even though it feels too simple. Display cables fail constantly, and they fail partially — carrying power and detection but not video, which produces exactly this symptom.
Use a different cable you know works. If you only have the one, try the other end in a different port before concluding anything.
Select the right input on the monitor
The monitor has its own input menu and it does not always switch automatically.
Press the monitor's Input or Source button and step through the inputs by hand — HDMI 1, HDMI 2, DisplayPort, USB-C. A monitor sitting on the wrong input shows No Signal and looks broken.
Force detection
Windows: press Win + P and pick Extend. If nothing, go to Settings → System → Display → Multiple displays → Detect.
Mac: System Settings → Displays, then hold Option — the Detect Displays button appears while the key is held.
Check the USB-C port actually carries video
This trips up a lot of people. USB-C is a connector shape, not a capability. A port only carries video if it supports DisplayPort Alt Mode or Thunderbolt.
Look for a small DP or a ⚡ Thunderbolt symbol next to the port. A plain USB-C port with no marking will charge your phone and move files but will never drive a monitor, no matter what adapter you use.
Watch out for the laptop's display limit
Laptops cap how many external displays they can drive, and the cap is set by the graphics chip, not the number of ports.
Base-model Apple Silicon MacBooks in particular have historically supported only one external display — plugging in a second does nothing at all. Check your exact model's specification before assuming a fault.
Reinstall the graphics driver
If it worked yesterday and doesn't today, a driver update is the likely cause.
On Windows, get the driver from the graphics chip maker — Intel, AMD or NVIDIA — rather than Windows Update, which tends to lag. If a recent update broke it, roll back to the previous version in Device Manager → Display adapters → Properties → Driver → Roll Back Driver.
Test the monitor on something else
Plug it into any other device — a games console, another laptop, even a streaming stick.
If it fails there too, the monitor or its power supply is the fault and you've stopped chasing the computer for nothing.