CUBASE 6.5 PROJECT SAVE PROBLEM

I have spent a lot of time on a Cubase project and have been saving as I go along. Now when I try and save it again a window appears stating:

The project could not be saved, because:
The project is corrupt!
The previous version of the project has been left unchanged.

This has happened on two other projects I have been working on. Can anyone tell me what is happening here and any solution?