Black line on Zenith 42 inch flat screen

I have a Zenith 42 inch flat hdtv.  For the past month or so, we have had a black line (about 1-1 1/2 inches thick) appearing from top to bottom.  It was on and off for a while but now it does not go away.  As soon as I turn the tv on its there.  Any ideas or suggestions on what could be wrong and how to fix? Thanks...