This TV would show the power up logo for about 10 seconds before shutting off. Normally, the logo shows for a couple of seconds before turning on. With backlight failures, the logo flashes briefly before the backlight shuts down.
TV’s not booting or stuck on the logo is usually a symptom of bad or corrupt firmware or a bad eeprom. If the firmware can not be reloaded, the only option is to replace the main board.
This model appears to be a Canadian model and used main boards are very scarce and expensive. A tip on badcaps indicated a 55LB6100 main board should work.
My exact model was 55LB5800-UG.ACCWLJR part# EBT63056403 and I bought a main board from 55LB6100-UG.BUSWLJR part# EBT62902106 ($45 USD). It fit and worked perfectly.