If you are using a desktop PC or external screen, it means you Can’t adjust your brightness on PC directly but rather by using your Desktop monitor navigation keys located most probably at the back or bottom side.
In the case of a laptop follow these steps
Update the display driver using driver pack or device manager. To do this press windows+x then select Device manager from the menu or simply windows+s and search Device Manager and click Device Manager from the results displayed.Once the Device Manager is open, Navigate to Display adapters then right click on display adapters and select update. Follow the procedure and that should fix your display problem.
The brightness slider setting is located at the action center for those Windows 10 version 1903 or later. For earlier versions of Windows 10 find the brightness settings here; Open Settings-> System-> Display, Move the brightness slider bar to adjust the brightness to fit you.