It sounds like your TV is in Shop Mode.
Shop mode is a set of picture parameters that have been optimized for a
store showroom or display floor. Shop mode does not allow you save
any picture settings such as brightness or contrast.
To get the TV
out of Shop Mode press and hold the Menu button on the front panel for
three seconds, the picture will blank momentarily then reappear with
"Dynamic Mode" shown on the screen. At this point the picture parameters
can be changed and saved.
If this method doesn't work for your TV there is an alternate method that might.
From the buttons on the front of the TV (not the remote):
1) Press the Volume + (increase volume) button
2) Whilst the volume button is pressed, press and keep pressed the menu button
3) Let go of the volume button and keep the menu button pressed for 5 seconds
4) After this time the screen should go blank and "Dynamic" will appear at the bottom of the screen.