I had exactly this problem the other day. It turned out to be conflict between my video driver's open GL display and carrara's. 
I had fiddled with the drivers in the control panel and it had the effect of switching the carrara default to "open GL". Then I got that annoying black screen that would light up again when I jiggled it with the pan tool. 
When I reset the default preview render to "software" all was bright and speedy again.