Exit Obsidian with any note, and re-open it.
Opening Obsidian results in the deletion of the content of the note displayed when closing the application.
Even with text in the Edit pane, the Preview pane shows nothing and changing the note o exiting the application renders the .md file completely empty.
Also beware of how you’re syncing your notes. Sometimes sync engines can get a little overexcited (or underexcited), and it can be complex to suss out sources of trouble. Watch for other devices that are syncing content, etc.