Project can not be saved - the project is corrupt

Not good. I have had to abandon using C9.5 because it tells me that all of my projects are corrupt, even after I have previously saved them successfully. I reverted to using C9.0 and the same thing has happened. I exported a wav mix file to prepare for mastering and then tried to export an mp3 version immediately afterwards to email to a collaborator and Cubase froze. I had to exit via Ctrl+Alt+Del and when I re-opened the project and tried to save any changes I get the ‘The project is corrupt’ message. What the hell is going on?