The mystery of the disappearing laptop screen brightness slider
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Updated for Windows 10.
I noticed that with the power turned off at boot time, I’d be unable to adjust my screen brightness. Instead of the usual 3 options (Turn off the display, Put the computer to sleep, and Adjust plan brightness), just 2 were available; i.e. Adjust plan brightness" had disappeared!
This is probably due to a bad driver (e.g., TeamViewer). The workaround is to restart Windows with the power plugged in, but this takes too long.
A better solution is: Windows + x: Device Manager: Monitors: (Select your monitor): Properties: Driver: Uninstall.
Post-uninstall, click Action: Scan for hardware changes. Your monitor should reappear, this time with the good driver installed.