I don't know, I went away for a cup of tea and a bagel and found it had restarted. Luckily I had saved all my work, but I had lined up all the open files on the toolbar so I knew what I was doing, because I have files like 'findings draft 1' and 'findings draft 20' and 'surely this must be the final findings draft' so I knew which ones I was working off i.e. the one on the left, the one in the middly bit of the toolbar etc etc :-(