My 55inch LG OLED tv model no '55EG910T'.
Screen just turned black while i was watching and the screen never came back, the voice is ok.
It displays white vertical lines when i swich it on.
No picture bt sound is ok.
In that case you don't have anything to lose by opening it up. Look for something obvious like caps with blown vents on top. That's been a pretty common issue on LG TVs. People only keep a TV for around 6 years nowadays on average so things are made as cheaply as possible to barely outlast the warranty, and caps are the one component that have the shortest rated life.
If the cable to the panel itself starts to come loose, you will also see vertical lines but on top of the normal picture. This is the cause of those wire bonding problems in all the CCD recalls where the ribbon (which looks like it's merely taped on) starts to come off from humidity over time.
As your panel can't even start up properly the most obvious suspect is the PSU. After all if the panel itself was dead you wouldn't even be able to see the white lines.