It can be a video-card malfunctioning (which can be checked by doing what fihart suggested), or a problem with the specific monitor.
Also if it is a problem of the monitor it can be caused by two things:
1) Got a pretty hard blow.
2) A manufacturing problem that caused it. (When I first got my monitor it has 4 pixels in the screen that were really annoying, a technician sent by the manufacturer came, took the faulty monitor and gave me a new one.)