External monitor isn't detected

Windows is not detecting the HDMI display, even after cable, port, and graphics-driver checks, so separate a display-mode problem from a failed video path. Power off, reconnect directly without a dock, select the monitor’s correct HDMI input, then restart. Press Windows+P and choose Extend. In Settings > System > Display > Multiple displays, select Detect, then press Windows+Ctrl+Shift+B once to reset the graphics driver. Test the monitor and cable with another source, and test the laptop with another known-good display; these checks identify which side is failing. Install Windows updates and display, chipset, USB-C, or dock updates supplied for the exact laptop model. If Device Manager still shows no second monitor, reinstall the display adapter driver and restart. A laptop HDMI port may use a different GPU, so use the manufacturer’s matched graphics package. If no display works from that port, hardware service is the safest next step.