I have no idea what would have caused that to start after a fall like you mentioned, but it's been my experience that whenever an LCD TV is darkening the picture during dark scenes, the culprit is the "Dynamic Contrast" mode.
I'm not sure what that mode will be called on your TV, but basically, in order to trick your eyes into seeing a wider range of darks to brights, the manufacturers have your TV dim the backlight when the image processor detects an overly dark scene. This grinds my gears to no end, because it washes out the colors when it does that. It's a stupid trick, in my opinion, and that feature should be immediately turned off from every TV. You might check to see whether this got turned on for some reason, and try it again with the feature turned off.