The second monitor work only sometimes?

The monitor and cables work intermittently, so first determine whether Windows stops detecting the display or detects it but sends no usable picture. When it goes blank, press Windows+Ctrl+Shift+B to reset the graphics driver, then press Windows+P and choose Extend. Open Settings > System > Display and select Detect; note whether the second screen appears there. Bypass any dock, adapter, or KVM and connect the monitor directly. Confirm the monitor’s input source and power remain stable. Install the PC manufacturer’s current chipset and graphics drivers; if the problem began after a graphics update, use Device Manager to roll that driver back. Disable Fast Startup for one test and compare cold boot, restart, and resume-from-sleep behavior. Try the monitor on another computer. If Windows consistently loses the display after sleep, collect the graphics-adapter model, connection type, driver version, and System event-log entries before escalating to the PC or GPU manufacturer.