There's something wrong with your file.
File>Export and choose IDML open that and try again.
If that doesn't work - create a new document. Go back to your old document. Pages Panel in the sub menu (top right) choose Move Pages and set the destination to the New Document.
Also: check, if the latest bug fix version of InDesign CS6 is installed.
The latest version is InDesign CS6 v8.0.2.
For checking see the "About InDesign" screen from the menu "InDesign" at the main menu bar (Mac OSX) or "Edit" (Windows OS; not exactly sure, currently I have no Windows version at hand).
Thanks. Tried the first option and saved as IDML, and then opened the new file. Still it crashed when I am trying to alter the Document Setup.
Then created new doc and moved the pages as described above. I lost the formatting on the basic paragraph style, but others came trhough well. Tried to alter the orientation - you guessed it - InDesign crashes.
Next I created a whole new document and simply copied and pasted the content across and now the document works! This is not obviously an idea solution if I had a document with dozens of pages, but in this case it worked.
Thanks, yes I have the latest version 8.0.2