DIAGNOSIS: The TV is on and running, but the backlight or the local dimming system isn't kicking in. This is a very common hiccup with this model, often related to a software handshake issue on startup.
FIX: Unplug the TV from the wall outlet, not just the power strip. Wait a full 60 seconds. While it's unplugged, press and hold the physical power button on the TV (if it has one) for 30 seconds to drain any residual power. Plug it back in and turn it on with the remote. If the picture returns, do a full software update via Settings > Support > Software Update to prevent it from happening again.
IF THAT DIDN'T WORK: Check your external devices. A stuck signal from a bad HDMI-CEC handshake (like from a game console or soundbar) can cause this. Unplug all HDMI cables, turn the TV on, then reconnect them one by one.
HEADS UP: If the screen is completely dead but you can see a very faint image if you shine a flashlight on it, that points to a backlight failure, which is a hardware repair job.