I've seen this a lot. The most common cause is a break in one of the signal wires (why is it always the blue one?) right where the video cable goes through the strain relief in the monitor case. I've fixed a few of these by trimming back the last couple inches of the cable and resoldering the internal connector to the fresh wire ends. It's not a pretty job and it is very time consuming. If i were doing it for paying clients i'd probably charge more than the cost of a new monitor.