I kissed the ground in gratitude when I found this thread. Thank you Laurie for your great diagnostic work and writing it all up in this forum. It saved me days, maybe even weeks of work.
I work on a Lenovo 520, which has two built-in display cards, an Intel HD 3000 and a Nvidia Quadro 1000M. I was playing video games during an interim, and updated the driver of the Quadro so I could play on the second monitor. That proved to be fatal when I returned to Cubase. Fortunately, all I had to do was roll back the driver for that video card, and everything worked again. But I shudder to think of the time I would have lost and the software mayhem which would have rolled through my system had I not found this thread.
Thanks!