Connected TV has become very dark

I have my iMAC connected to my Sony Bravia for months with no problems and all of a sudden today the screen has become very dark when I try to play videos on it.

The TV settings themselves have not changed, I've checked and the brightness levels are high.

I can't figure out what could have happened.

Where have you checked the brightness settings? on the TV?

It almost sounds like perhaps the color profile changed.

Check under Preferences -> Displays -> Color and see what profile is selected, or try to set a custom settings with the Calibrate function and see if you can improve the brightness...

Maybe you have checked this already but is the TV darker when it is switched to other inputs?