1. Unplug ALL connections, USB drives, etc. Test.
2. Unplug power, wait 20 or so minutes, apply power, test.
If that doesn't help it's in need of repair. The video and story does NOT tell you which board or such. never has.
I have an old LG 42LW659S which out of a sudden is caught in some kind of bootloop.
It starts only into the LG logo screen, then dims until the screen goes black, enters stand-by and starts again. The whole cycle takes about 30 sec.
I have made a video: https://youtu.be/7FmywuVjYEQ
Looking forward to any response.