Hi @mohua ,
With the laptop on and a display showing on the external monitor, shine a torch at an angle close to the laptop screen to check if you can "see" an image at all.
If you can you have a backlight problem.
This could br caused by a faulty backlight power supply from the motherboard, faulty backlight cable (not sure if it is a separate cable or part of the video cable), a faulty lid switch or a faulty LCD panel.
If you cannot detect any image at all (try in a darkened room, as it may be easier to see), then you may have a faulty video cable, faulty video cable connections at either end (motherboard or screen) or a faulty LCD screen.
Here's a link to the service manual for your laptop which may be of some help.