Which PC are you using? As Harjit implies the most likely reason is that you have a function button lock that you've accidentally enabled so it now works just as "F2" not "adjust brightness"
I found Fn Lock at laptop and just clicked it - I probably clicked it before acidentally.
(If I click it again just to test the brightness control returns.
Windows OS
This topic area includes legacy versions of Windows prior to Windows 2000: Windows 3/3.1, Windows 95 and Windows 98, plus any other Windows-related versions including Windows Mobile.