There is no user serviceable area I know of for that issue. Most folk arrange for repair. Read this recent discussion if you are a TV repair tech.
If it was that far out of warranty I'd do that but I'm an electronics designer and more. If you don't do this work, there are LETHAL voltages inside and you do this at your own risk.
My Samsung LN46c600F3FXZA has a distinct dark bar about 1/6th the area of screen. You can still clearly see the image, but everything within the area is darker.
I doctored a stock image of my tv to simulate how it looks.
any clues to what this is, can it be fixed do-it-yourself style, and how much the repair might cost?