Mark, this normally means your project has corrupted.

If you have previously uploaded your project with the inbuilt FTP, there should be a backup.

Using Windows Explorer, navigate to the project and look in the backup folder. If any files exist, copy the most recent up to the project directory. In the Project Directory, rename project.iwprj2 to Broken.iwprj2, now rename the backup file to project.iwprj2

With luck WX5 will open your project.