I'm sorry this happened to you. We have not had any reports so far of notes disappearing or not getting saved so I'm not aware of any open issues. You say that an error message appeared: did this happen while working in Tropy (before you closed it) or when you opened Tropy again? My first guess is that there was an error when Tropy saved your notes, the error appeared, the note was still visible (but was not saved) and you closed Tropy (at which point the note was lost). If this is what happened, we will need to know what the error was in order to debug this.
In order to test if this is reproducible on your computer, you could create a new project and just create a few notes for testing. Tropy saves a note after a short delay after you stop typing in a note. That is, if you are writing a few notes, Tropy is repeatedly saving them.