Hi Nitish,
Are you sure auto-brightness is not activated?
Perhaps energy saving is enabled?
To test everything with your screen i suggest you launch the test menu and see if everything works there, there is a special button called "dimming" there to test as such;
-> go to your "keypad" (as if you want to call somebody)
-> Type in: *#0*#
It should start the test menu instantly.
Let us know what the tests do :)