Everything was working fine until I tried to change the brightness once. Then whenever I plug in/out power to the laptop Brightness of the screen sets to zero.
I also tried to change the brightness from System Settings/Brightness & Locks but found out No Slider there.
I could only reset brightness manually by running
$ sudo su # echo 150 > /sys/class/backlight/intel_backlight/brightness
I tried to change Dim screen to save power option too but it did not affect anything.
Also the Brightness control keys do not work once power is changed.
Everything works as normal once I manually adjust the brightness to 150.
What is the problem?