Acer TV Plasma screen problem

Question

a black thin band has started to drop from the top of my acer TV screen, it runs across the top of the screen and drops down the the middle . it is starting to get worse, does any one know what this is & how to correct it , all other aspects of the TV work fine and the black band is not a physical thing it is part of the viewing section of the screen.but should not be there.Thanks