Plasma screens work similar to a neon light. Each pixel has it's own voltage supply and they are all grounded together. A ground wire came loose from the top section.
It could be as simple as finding your zoom button on your remote control or you should check that the television is not near any magnetic fields like large speakers.
I am sorry to tell you that your screen has been damaged. Either excessive pressure or more likely high heat was exposed to the plasma screen. The cells are now non functional and never will be.
There are a lot of possible answers on what could cause that problem. One of them is that your plasma screen is burned out. I would check that out first.
