Have you made sure your cables to your screen and your graphics card are properly connected? A loose cable can do exactly what your describing. Do you have anther cable you could try to use as well? It might even be the cable itself...
And if it's not a loose cable - it could either be your screen dieing, or possibly your graphics card dieing.
Thanks. I've checked the cable connections, but maybe I should just go ahead and replace them. The graphics card suggestion also makes sense. JB