Suddenly can't open project file anymore. (used by another process or Storyline just crashes)

There are no updates pending and I've tried restarting several times.

I was working on the projectfile yesterday and everything was working fine - but today whenever I try to open it it either tells me the file is already being used by another process or Storyline just crashes. 

I've tried making copies of the files I need to work on - but that didn't fix it either. Storyline just crashes with a "no response" error whenever I try to open them. I'm really at my wits end here.