The third time it happened, I had to fully uninstall the graphics driver, delete the software, and uninstall the default driver.
Switching to generic PnP monitor restored the brightness, and the brightness stayed up when I re-installed the graphics driver. The second time it happened, I had to fully unistall the graphics driver and delete the software. The first time it happened, I updated the graphics driver (Intel HD 4000), and that fixed it. Ever since I “upgraded” from Windows 8.1 to 10, the brightness adjust buttons immediately make the screen change to the lowest brightness setting – unusable in all but near-darkness.
I have a Lenove Ideapad z400 that came with Windows 8.
