The first aid for most software problems is to reboot the computer.
(1) Is it job specific? Can you open one of the example jobs supplied with CAESAR II installation and reproduce the error? Trying this on a different machine may not make a difference: if "another" machine was purchased at the same time as the "first one", the chances are the system configuration will be identical…
(2) On closing the graphics, the CAESAR II tries to update an XML file with graphics related settings (colors, zooming, etc.). Try deleting the XML file (in the Job directory with the Job name) and then plot again.
(3) OpenGL implementation on many video cards is far from superior and may cause all kinds of problems. Go to Tools -> Configure/Setup, Plot Colors tab and clear the "HOOPS Graphics use OpenGL" check box. Then Exit w/Save and try to plot again.
(4) Check the video card manufacturer’s web site for the latest drivers available for your video card.
(5) Check your video card performance compatibility with the HOOPS graphics at [http://developer.hoops3d.com/graphicscards/graphicscards.html] or [http://www.solidworks.com/pages/services/VideoCardTesting.html]

If nothing of the above works, we will need to setup a "buddy help" session to "witness" the problem and try to resolve it (call 281-890-4566 to schedule an appointment).

About working in the spreadsheet with the graphics window open:
from the Piping Input click the Graphics button, at the Graphics window click the "View Input Spreadsheet" button on the toolbar to bring the Piping Input again. Clicking the same button again will toggle the Input screen on and off. If the button state is already "pushed on", then you can access the input spreadsheet by clicking the appropriate icon on the task bar.