Of course this should never happen. If you can figure out how this happened it would be a high priority to fix, but we'd need steps to reproduce as per the bug report guidelines. As with any digital work you should keep regular backups, so restore a recent one and continue working. If you don't have a recent backup, you have learned a lesson in why you need backups, and you can send your project to ashleydal@scirra.com where I can try to recover it, but I can't guarantee anything.