If you accidentally delete an important note or entire folder, you can try to restore it.
Firstly, go to Preferences → Backups.
Then click Browse Backups... button. You'll see list of your backups. Choose the one you would like to restore and click Restore... button.
Now, you need to choose which folder(s) you would like to restore and finally click Restore.
And that's it. Restored folder should appear on the bottom of the folder list.