THIS IS WHAT FIXED THE ISSUE FOR ME (Date: 23rd May 2018):
- I opened Device Manager.
- Noticed that Display Adapters section was reporting problem (showing exclamation sign) for the "Intel HD Graphics 620". (This solution may work for other onboard graphics too).
- I right clicked on that and chose "Update driver" option.
- On the pop-up window, I chose "Search automatically for updated driver software" option.
- Windows searched and automatically installed the driver software over Internet and the Brightness Controls started working again immediately (without restarting the PC).
- Device Manager also stopped reporting problem with Display Adapter.
I hope this helps some of you if not all of you.