rendering problem - render completion render goes black

I have a major problem with Autocad 14 rendering. I have a scene that I am rendering (using the sky environment and medium/high settings). The rendering seems to proceed OK, but upon completion, the whole screen goes black apart from a square in the bottom left of the window that seems to show just a portion of the sky. The render took over 17 minutes.

Please see the attached image. I need a solution to this asap. Has anyone seen this before or can anyone help?