My mum had this issue. Could be a loose connection on one of the vertical strips that run down the screen. You can access them from the back and disconnect/reconnect the one corresponding to the position of the vertical line. There are youtube clips of the procedure. I did it with zero technical knowledge and managed to fix the issue.
My Panasonic Plasma is 2007 and will not DIE either. 12 years and the damn thing REFUSES to give up the ghost. I see all these great new TV's in the showrooms and am tempted but I keep thinking...nah I'll wait a little longer and surely my Panasonic will one day just not turn on. Still waiting.