Hi! I had this problem too for a long time. Mine did not occur after a windows 10 update. I updated my gpu driver and then it stopped working. Here is the soultion:
Go over here and download the tool https://www.guru3d.com/files-details/display-driver-uninstaller-download.html
Then uninstall the nvidia driver using the tool (you can check tutorials online if you dont know how)
Afterwards go to nvidias official site: https://www.nvidia.com/Download/driverResults.aspx/141908/en-us and download this latest driver
After that the prolem is solved! It seems that the nvidia driver interfeared with the backlight.