When you launch a video playback app and then drag the app’s window from a primary monitor to an external monitor, video playback may stop or may display a black screen. This problem may occur on a Windows 10 device that has both an integrated graphics processing unit (GPU) and a discrete GPU and is configured to display on an external monitor in Extended mode.
Due to an architectural limitation, some video apps are not able to move across different monitors that are connected to different GPUs.
Once the video player is on the desired monitor, restart the video playback or close and restart the application and video playback should resume.
You can also try connecting the external monitor to a different display connector, such as switching from an HDMI port to a DisplayPort, or vice versa.