I noticed this problem in the last 2 weeks or so, but searching around the ‘net returned no results.
So I noticed that if the power plug turned off during bootup, I would be unable to adjust my screen brightness. To resolve the problem, a restart — with the power plugged in — was required.
Right-clicking the Power system icon — located on the bottom right of the task bar, clicking Power Options -> High Performance -> Change plan settings showed just two options: “Turn off the display”, “Put the computer to sleep”. “Adjust plan brightness” had disappeared!
The solution is:
Start -> Control Panel -> Hardware and Sound -> Device Manager.
In the Device Manager window, look for “Monitors”, then right-click your monitor and select “Uninstall”. Be sure to also check the “Remove driver” option. Once removed, click the “Scan for hardware changes” icon.
Voila. “Adjust plan brightness” returns, mystery solved.