Hi,
Here’s a composite image from the [https://servlib.com/lg/tv/60lm6450-chassis-lb22e.html|service manual] for the TV that shows how to check if the problem is in the mainboard or the power board.
[image|2204499]
(click on image to enlarge for better viewing).
You will need to have a DMM (digital multimeter) and know how to use it safely when testing for voltage when lethal voltage is also on the same board, to find out which board is the problem. If you haven’t got a DMM (or know someone who has) then contact a reputable, professional TV repair service and ask for a quote