Can't 'Save As' in Illustrator

This has only happened in the last few weeks, I was using Illustrator fine up until then. However, now I cannot 'Save As' any Illustrator file. I can 'Save a Copy' and I can export but when I click 'Save As' the screen locks up and nothing happens, no window appears to allow me to choose a location.  My only option is to press escape or close Illustrator altogether.

This is making life extremely difficult as I'm having to save everything as a 'Copy'.

I have updated to the latest version, I've checked my graphics drivers are up to date and I've closed and restarted both Illustrator and my computer multiple times.  Has this happened to anyone else? Any advice on fixing?