I'm working on a file imported from AutoCAD (created by someone elese, badly managed though).
Sometimes editing block (removing block within a block by 'explode' command) makes the WHOLE DRAWING (every single element, even the edited block) disappear.
I have to save a file every 2 minutes and while it happens I re-open the file without saving it.
What build of Rhino? Can you try the current available build, which is 5.1.20808.315, 8/8/2012?