Today I faced a very peculiar issue, we had a display driver update for our Lenovo Laptops Z500 and suddenly the brightness went to 0 no matter what we did, nor even the reboot resolved the issue.
finally this registry setting did the trick.
Changed the dword from f048 to ffff
or whatever the value just change it to ffff
rebooted the laptop and all was fine and functional again. lot of Lenovo laptops of even different models are facing this brightness issue.
just open regedit and find FeatureTestControl
take the backup of the registry settings before making these changes. always do a backup. one day you will thank for this advice.
maybe this helps someone somewhere.