Storyline wil not play or preview slides containing video

I am really confused. I created a course recently which contained a number of video clips (some as screencasts recorded from within Storyline). I published and exported the course fine.  I have now gone back into the course to make some edits and none of the videos will play - either in storyboard mode or in preview. I just get the rotating circle.

All the source material is standard mp4 files and I have even tried creating new screens and imposrting a known piece of video.  It is as if Articulate went to sleep and when it woker up decided it didn't like MP4. 

Any thoughts?

1 Reply