Hi, I'm John! I will help you with this.
This usually happens after a display driver update, Windows update, or dock/adapter reset.
In the meantime, if you haven’t already already:
- Right-click the desktop > Display settings > scroll down > click Multiple displays > Detect.
- Also press Windows key + P and choose Extend.
- If the third monitor is still not detected, restart the PC with all monitors connected. If you use a docking station or USB-C/HDMI adapter, unplug it from power for 30 seconds, then reconnect it.
Next, check the graphics driver:
- Right-click Start > Device Manager > expand Display adapters > right-click your graphics driver > Properties > Driver tab.
- If Roll Back Driver is available, try that. If not, go to the PC maker’s website and install the latest graphics driver for your exact model.
Let me know how it goes.
If issue still persists, to help narrow down the issue:
- Is the third screen connected directly to the laptop/PC, or through a dock/adapter?
- Does the monitor work if you swap cables or connect it as the only external display?
- What update was installed before this started, Windows update or graphics driver update?