If so I doubt you'll get an answer but did you try unplugging it from the wall? My older LCD is not "black" either.
Got this strange problem with my brand new smart tv 6305.
A couple of hours or so, after tv is turned off, the screen becomes grey. Seems that some kind of lights behind the screen are active ? When powering the TV on and then off again, everything is normal for a couple of hours or so, then back to the grey screen.
Anyone have any idea, to what causes this ?