Thanks for your replies. Unfortunately I just had to revert to an earlier saved version which seems to be fine, although of course, old.
Nothing I tried fixed the problem - all of my 3d views (saved custom views or generic) just opened up a white, empty window. Also when I'd create new 3d views - same problem.
I'm using version 10 (so I don't think this was the problem: pressed command+Y, which in ArchiCAD 11 activates '3D Cutaway')
and I couldn't find any other posts from people with similar problems that were helpful.
But thanks for the suggestions.