Screen went white or gray

I wonder if any one has any idea why my Dell Pent.III moniter screen went all white or gray when we had shut the system down for a break. After we would restart it, it would run ok then when we would click on something it would all go white or gray again. We contacted our Dell tech. who helped us run some tests with the Resource CD, then they were to get back with us, which they never did. But as it turns out we throughly scanned all disks and defraged and it seems to be working great now.