Mistakes happen. Sometimes things go wrong and you need to make things as they were before.
Every time you close ReadCube the ReadCube (legacy) desktop app, it keeps a backup of how it exists in that moment. When you reopen the app and close it again, another backup is made. The third time the app is closed, the new backup which is created pushes the first one out; at any given time, ReadCube keeps backups from the last two times the application was closed.
If you need to restore ReadCube from one of those two backups, here is how to do so on a Mac.
To restore from a ReadCube backup on Mac, please start by fulling quitting ReadCube. Once it's closed, launch your Finder and hit "command + shift + g". This will pull up a "go to" field.Type in ~/Library/Preferences
Once there, look for a folder called "com.readcube.Desktop". You'll see a folder called "data". Within there, you'll see two back-ups and one database file. They are marked by ".db" for database and "backup" for backup. It is possible that you'll see several .db files and several .backup files. For this, please disregard all but the first, as in this screenshot:
Delete the .db file and change the ending of one of the "backup" files to be "db".
This will make one of those backup files your current database.
Now, re-launch the ReadCube legacy desktop application and it will load with the last backup!