I had a similar issue. LG TV went blank but when I shine a flashlight on the screen I could see shadows. when I first turn the tv on I could see the LG sign and then the tv goes blank. Later I found out that 2 of the 3 backlight- strips went bad. I assume if one strip goes bad the power board inside the tv will detect that, and cut the power off to all the light stips.
It could also be that the power board is not sending voltage to the backlight. if you have a voltmeter, you could easily check the power output on the main power board. if you don’t have output voltage, then your board is bad. I found used one for $30 on Amazon. if you do have output voltage then your backlight is probably bad. backlights are cheap and can be easily replaced.
when attempting a repair, make sure that you don’t damage the front screen as it is fragile and can be easily damaged.
find parts using the TV’s model number and the number of the part that you want to replace. part numbers can be found on the actual parts.