Word for Mac won't save .docx format

Basically I was working on a .docx document,  I can save this document as a .doc but not as a .docx.  When I try it appears for a split second on my desktop (or wherever I try and save it - server - external drive) as a  temp file but then just disappears again and the document I am working on just stays the same, doesn't get modified at all (for example when i try to save it out as different names).

I can save it out as a .doc but I don't know how long even this will last as there was no warning when it stopped saving to .docx - I just lost a whole days work - despite saving every 20 - 30 mins.

I have tried copying and pasting into new documents and saving them out as .docx I have tried saving as a .doc and then copy and pasting into a new document and resaving as both a .doc and a .docx  nothing seems to work.

This started happening to me last week but started with other people in the company a couple of weeks back. there has been no crossover of documents between our computers so a virus seems unlikely.