Hi, I'm Anvar, I'd be glad to help you today on this
It usually get fixed by refreshing display and Monitor drivers
Right click windows button and click device manager.
Expand Display adapters section by double click on it and right click each item listed there and click uninstall device (no need to select remove software option)
Then Expand Monitors section and uninstall the monitor listed there then restart computer and check brightness slider
Hope this helps. Let us know if you need further assistance.