sounds like a backlight issue to me, of some sort. given that you can still see a picture, its only dimmed not completely not working (not working it would be pretty much black). as for the exact cause.. not sure. perhaps zone backlighting issues? a problem with the hardware controlling backlighting?
if the image is discernible on the lower half close to center but not at all near the edge, then its likely a failed portion of the backlighting system and bleeding over is the only thing letting the image be viewable. i'm guessing you have a grid array type or at least top/bottom edge system going on... as ccfl tubes are almost nonexistent anymore and edge lit on the bottom would not do this. http
/www.cnet.com/news/led-lcd-backlights-explained/
you can try running your tv in game mode to disable any auto-dimming and set the backlight higher to see if it affects the brightness of the lower section.. though thats just wishful thinking.