Hi there rajat,
I would like to ask for some more information. Are you able to hear the drive or the fan spinning while the screen is black or not? If you can I would recommend connecting the laptop to an external monitor to see will there be any video signal going out from the laptop. If it does go out, then there's a need for LCD inverter replacement. If there's nothing showing either on external monitor nor display, but the fans are still working, the problem that you have may be due to the failure of the video chip. Sometimes, after much of a use and less cooling the graphics processor which is responsible for video signal transmission get unsolders, the thermic paste goes away and it gets unsoldered, therefore the screen doesn't show anything. If you possess knowledge of notebooks' hardware you can change the paste yourself by opening the laptop and check if the chipset that is next to the processor has got unsoldered or you can bring your laptop to a technician to get it soldered again.
Hope I gave some ideas. Cheers.