_Invalid Project File messages in Cubase tell you that you have a problem, and that problem may be that you've just lost your project. So - if you don't autosave your projects, start now, if you don't manually save as a new version (ctrl - alt - S) before you make any radical changes or after you complete a task - start now.  This stuff is all in the Manual.  Oh, and keep a note of what is what!