So sorry for the snag!
Are you running the latest version of Mockups (3.3.9)?
We recently added a new backup system of all projects locally to an internal folder (starting with version 3.2.1). The way to access the local store folder where these backups stay is described here.
Also, were you saving the file locally or on a network drive? We strongly recommend to keep a local copy of the file, especially while working on it to avoid any issues.
Sorry again for the hassle with this. Please let us know how this goes, we’ll dig deeper if needed!