Sorry to hear this. Sorry I’m not up to date with the latest syncing-related bugs or fixes, so I’m not sure if this will help.
Have you attempted to use the File Recovery plugin? It is enabled by default. Go to the preferences, and “File recovery” section. There, you’ll see “Snapshots” and a View button. From there you need to type your file name to see the available snapshots. Hopefully something will be there.
As for a cause, I think there needs to be more info. Are you using any sync features? Is your vault located in any folder that is syncing from the OS? An iCloud folder, for example?
What other 3rd party plugins do you have installed?