It does not happen very often, but some times a lone pixel on an LCD screen can get stuck “on”.  When a pixel is stuck on, it usually shines a bright white or green colour.  When a pixel is dead, it does not shine at all.  A dead pixel cannot usually be fixed, but a stuck pixel can… some times.  To get instructions, check out http://www.wikihow.com/Fix-a-Stuck-Pixel-on-an-LCD-Monitor.  We have tried these suggestions a few times, and it almost always fixes the problem.