LED TVs, as the name suggests have LEDS to illuminate the screen, this section inside the TV is called "BACKLIGHT"
Every TV brand and model has a different configuration of LEDs.
All LEDs will eventually either wear out slowly due to hours watched or excessive backlight brightness level, a setting which can be adjusted in "Settings"
Worn out LEDs can change colour and turn blue, in patches or all over the screen.
Or fail completely.
Some TVs will not function at all with a faulty backlight. eg start up and then shut down, as Sony will flash a 6X error code; Panasonic 1X etc after initialy trying to start.
Some models still function with out showing a visible picture, the screen is actually still on but cant be seen, shining a torch at the screen can show this fault.
In general LEDs can be replaced, usually only a full kit replacement will result in a satisfactory and long lasting repair.
Single LED replacement will fail again within a period of time, as all the LEDs will be worn out at the same rate.