Unable to adjust brightness in my windows 8.1

I am having this problem from past 3-4 days I am unable to adjust my laptop's brightness. The functions keys for brightness won't work. I also tried changing the power settings to high performance and balanced, that also doesn't changes the brightness.
I also tried manually adjusting the screen brightness. The slider moves but the brightness is not changing.

I am using IntelR (HD) Graphics family.
I tried changing it back to Microsoft basic display adapter but that also didn't help.

I got windows 8.1 installed from the factory itself and everything was working fine until past few days.

Any suggestions??

P.S. => I am using a Lenovo G50 series laptop

First off, figure out whether or not your laptop has an ALS (Ambient Light Sensor) which will automatically adjust the brightness (if all goes well). Usually it won't override a manual setting, but it's kind of up to the developer which one takes precedence. As for the keys not working, you probably removed (or didn't install after a Windows reinstall) the helper software that controls these functions.