do we need to replace logic board to get backlight

In this case, connecting to an external monitor wont give a big clue about the problem. It's clearly a lightning issue, not a video issue. To those who are reading this, be aware that when you loose the backlight while you're moving the laptop lid up and down it means that your inverter cable has a problem. Can be a loose connection to the sockets (to the logic board or to the inverter board) or a damaged inverter cable. When you have this lightning behavior don't wait too long before checking the contacts or replacing the inverter cable because if the cable cut in the hinge and short there it will cost you a logic board instead of a cheap inverter cable. I think that's what happened to goutham. If there's no voltage at the inverter socket output on the logic board then the logic board is probably faulty (shorted) before the socket.