@alastair said:

Hi, Hope somebody can help.

.......I do believe that the imported drawing was maybe corrupt....

Working with the imported dwg could mean that you modeled far from SketchUp's origin, as mentioned by Box. Import without preserving the drawings origin. This puts it right where SketchUp's origin is.

Also get rid of guides. Having several in your model can also be your "orbit -> vanishing model"- problem.

And once you do have your model on screen as desired, do save that scene. Hitting that scene tab brings your model back if it gets lost again.