DIAGNOSIS The backlight array has failed. It's a common hardware fault on these LED panels.
FIX Unplug the TV. Press and hold the physical power button on the TV itself (not the remote) for 20 seconds. Plug it back in and power on. If the screen is still dark, the backlight LED strips need replacement.
IF THAT DIDN'T WORK It could be a failed T-Con board. That's the small circuit board connected to the panel. It controls the image signal. Requires disassembly to check.
HEADS UP Backlight repair means removing the entire LCD panel. One crack and the TV is totaled. This is a pro job.