Most likely the edge lighting has gone bad in that area. It may be fixable at a TV shop, but will probably cost as much as a new TV. Take it to GEEK Squad at a Best Buy for a quick diagnosis. Then if it cannot be fixed for a reasonable price, they will recycle it for you and you can look for a new TV.
Sounds like the fl tube inverter board power is failing. All flat panel screens us thin fluorescent tubes to light the edges of the screen, and when the tubes fail either from age or due to a power issue the screen will go black.
