Diagnosis
This usually points to the LED backlight system failing. On these TVs, the LEDs are arranged in strips behind the screen, and one or more strips can dim or die, causing an overall dark picture.
How to Fix It
First, rule out a settings issue. Press the 'Settings' button on your remote, go to 'Picture', then 'Advanced Picture Settings'. Make sure 'Eco Mode', 'Power Saving', and 'Ambient Light Sensor' are all turned OFF. These features automatically dim the screen to save power. With them off, set 'Backlight' to 100 and 'Brightness' to around 50. See if that fixes it.
If That Didn't Work
If the screen is uniformly dim (not just dark in patches), the power supply board might not be sending enough voltage to the LED backlights. This is a component-level repair and isn't a DIY fix for most people.
Heads Up
Don't leave static images (like a news channel logo) on screen for hours on end. This can cause 'image retention' which can look like dim spots, but it's temporary. Run a built-in pixel refresher if your TV has one, or just watch varied full-screen content for a while.