Horizontal Black Line on Plasma Screen. On my Panasonic TH50PX77U, there is a horizontal black

Question

On my Panasonic TH50PX77U, there is a horizontal black line through the middle of the screen. It shows up on every channel and input. If anyone could please tell me what the problem is, I would really appreciate it.

Thanks in advance.

Samsung LNT4661F vertical line growing over time down screen, another is starting.  I can see the line when the TV is off, too.  It just appeared one day, and has been slowly getting longer.  I only have a few weeks left on my warranty.  Repair person said it was external damage and our fault.  I don't think that was the case.  HELP, PLEASE!