DIAGNOSIS This sounds like the TV's ambient light sensor is turned on and is aggressively dimming the picture based on your room's lighting. It's a feature meant to save power and reduce eye strain, but it can overdo it.
FIX Press the home button on your remote, go to Settings > All Settings > General & Privacy > Power and Energy Saving. In there, turn off "Ambient Light Detection." Also, set "Brightness Optimization" to Off. Now go back to your regular picture settings. You should see the full brightness range available again.
IF THAT DIDN'T WORK Make sure you're not in a special picture mode like "Eco" or "Movie." Go to Settings > Picture > Expert Settings and check that "Contrast Enhancer" is set to Off. Sometimes these "enhancing" features actually crush the contrast and make things look flat.
HEADS UP If the screen looks uniformly dim and grayish even with those settings off, it could be a backlight issue. Run the built-in picture test in the Support menu (Settings > Support > Device Care > Self Diagnosis > Picture Test) to rule out a panel fault.