Unable to decrease brightness!

I'm unable to decrease brightness in my lenovo thinkpad T550 after the replacement of the lcd screen. Even the function keys F5 & F6 doesn't work. Please help. 

Did you replace the screen with a genuine Lenovo branded item or some generic replacement.

Have you tried adjusting the brightness in Windows.

I replaced with a compatible part only & the brightness bar is nowhere to be found. 

Sorry but what do you mean by "a compatible part"

How are you checking for the brightness bar.

Like through their sites list of compatible parts kinda refurbished one.

Brightness bar via settings > display.  

Still doesn`t answer the question as Lenovo do not sell refurbished components, had you purchased a replacement screen direct from Lenovo it would be the exact screen that you need, out of the box new and be a well known brand such as Panasonic or Samsung, that said the screen works so the problem could be with Windows.

What version of Windows do you have, 7, 8.1 or 10.

Lenovo replacements screens

Windows 10.

In the Display settings do you see a Night light setting.