the fact that the project was opening from the back up does only mean that the back up was not corrupted (or did you create your back up right before the issues?). Since Cubasis saves your work on the project file automatically, the project that made Cubasis fail to open was in a different state.
So if your project does not open, you can do a backup via iTunes and send the project file to us. The more precise information we have about the state in which you had your problems, the easier it will be to reproduce your issues (for instance, how many events did your project have, what effects did you use, how long was your project, …).