It sounds like there is an issue with the ribbon cables that attach the plasma display to boards that control that display. If you remove the back panel of your TV you will find two circuit boards with several ribbon cables attached. One board controls the horizontal display the other is for vertical display. These boards may be hiding under speakers or other components but will likely be near the edges of the TV. I would unplug and re-plug in all the cable to check that they haven’t come loose over time. If they are damaged you will have to source replacement parts.