Laptop screen image is truncated on dat...

Our office Dell 3400MP data projector has always worked perfectly with our Dell laptop (Windows XP) until after a training session by a non-techie external trainer the other day, who may have pressed a few of the wrong buttons when closing down. When I set up the equipment for another trainer the following morning, the LCD image only showed two-thirds of the desktop screen. The projected image was square rather than rectangular, but it wasn't that the image was squashed, rather that a vertical band was cut off from sight.
We borrowed another data projector and it worked perfectly with the laptop, which indicates that it was our data projector which was causing the problem.