cannot save doc

I was just working with a very short doc. It was sync with MS Word, and I was adding things on it with Zoho writer. As soon as I started (well, 1 or 2 mins into my work), it starts saying 'we cannot save this doc .... please copy it to prevent date loss' (sth like this). What do you mean???? I only know that means I cannot work on Zoho! So frustrating. :(