shows the LG logo for 2-3second then the screen goes black

I’ve have an LG 42 LB5500 it turns on but only shows the LG logo for 2-3second then the screen goes black, ive search net to see if I can get some info but I’ve got mixed reply’s saying it’s either the LED backlights or the main board, I’ve stripped the TV to check the backlights & when plugged in with the screen off all the LED backlights light up but only for 1-2seconds but all of them do come on, does this mean the LED back lights are fine & it must be the main board, any help is appreciated Thanks Lee