"document not saved" error

From time to time I try and save a workbook and all I get is, after excel
thinks for a while, a msg box saying "Document Not Saved". While this is a
good contender for the prize of 'most uninformative error message ever', I am
left with no way to save my work.
I can't quite tell what sets this off, I thought it might be system
resources but I've saved much larger spreadsheets in the same session as
being completely unable to save my original one.
Sometimes copying and pasting each worksheet individually into a new
workbook works, sometimes it doesn't.

Can anyone help?!?