Please stop and restart netViz and re-open the project. If this doesn't resolve the issue, please go to a back-up of the project.
If the problem persists, or you do not have a recent back up of the project, please submit the project to Technical Support for possible repair.
The only way to prevent total loss is to have a good back-up strategy.
Since all NetViz files are stored in their own directories, backup can be easily coordinated through your Windows Systems Administrator.