nec 61" plasma

I have a 61" nec plasma, i've had it for about 2years. Just the other day i turned it on to watch tv and only half of the screen was working. I currently have a HDTV cable box, and a dvd player attached separatley to the tv. when i watch either tv or a dvd only the bottom half of the show is visable. anyone have any idea what the problem could be?????