If the note is still open and you haven't switched away from it, you can use Ctrl/Cmd-Z to undo (probably after selecting all and copying the new contents to a safe location). Beyond that, depending on when exactly you created the previous note contents, you may be able to restore from one of the automatic backups — you can look at the timestamps of the files to get a sense of when they were made and think about when you last edited the note before this change. If you find the note contents you want, copy it to an external text editor or word processor for safekeeping before syncing.

And to clarify, no, if you add an item to another collection, it's just the same item. See The Zotero Collections Model to better understand this.