to add to what bruno21 said, create a new file, and see if it happens then. If not, add some small item, see if it happens. I usually lose my view screens when my video memory begins to run short. Especially true if the it is anything but wireframe selected. To compensate, I have to divide the image into layers and click them on and off when I work on the scene. (512MB video Ram, 1GB Ram- pushing the Macbook Pro as far as it will go, and having a wonderful time.)

...Insert some witty or thought provoking comment here...