I'm sorry to hear that you're having this issue.
Unfortunately, I'm not able to make a diagnosis based on something I can't see. What I'd recommend is to have your unit serviced by a technician so that they'd be able to identify the problem.
I've searched the threads, but been unable to find someone with the exact same problem. I've had my Samsung 37" LCD TV LN-37A550 for right at a year and a half, (yes, out of warranty), when out of the blue, the screen has gone to about half brightness. You can see the picture, but any type of dark scenes are very dark, and the picture is not vibrant at all like it was just the day before. I've used all the settings in the menu with no luck. I took the back off the unit and can see the backlight is working, but don't know if it used to be brighter. Just looking at internal components, I don't see any obvious failures. Any ideas?