"Invalid project file" issue

Cubase, win 7:
issue opening session which was recorded couple of days ago, (no crashes on saving or so) - today we just got the “invalid project file” message when musicians came to the studio for another record session.
All other 11 projects created the same day with the same settings opens normally.
Any known solution for that?
Working with cubase since version 4, never had issues like that. System stable.