The company file is just a regular data file on your hard drive. It would only disappear if it no longer was accessible in the file system. Typical situations of this would be if the file was on an external drive of some kind (USB stick, network mount, external hard drive) and the other media was removed/disconnected. A much less likely situation is that your hard drive is experiencing problems - bad sectors, failed media, some virus messing up the permissions, etc. and thus making the file no longer accessible.
You have you can search for the file in the FInder's Spotlight. What to search for depends on the version of QuickBooks. For QuickBooks 2016 search for .qb2016 For 2015 search for .qb2015. Etc. That should show you the file if it is on your hard drive, in a place Spotlight searches, and if your Spotlight search indices are working correctly.
By default QuickBooks places a new file in the user's Documents folder. If you didn't specify a different location the file should be in there.
Finally, you should never run QuickBooks without having it automatically create backups on exit. We tried to force people to backup by defaulting that to be "On" and requiring them to specify a location. The best location is an external cloud "drive" like iCloud. That way your data isn't just on your fallible hard drive but also stored on Apple's servers.
If you have backups on you can go pull the latest one and proceed from there.
