If you accidentaly deleted an important note, don't worry. It will be not hard to restore it in a number of ways:
 

1. From Recently Deleted option in the notebook menu

When you delete notes, they aren't actually deleted from the database, but just marked as deleted.
 
It means that you can easily restore them using "Recently Deleted" option in the notebook menu.
 
If you want to get rid of all your deleted documents, you should create new notebook, drag all notes from the old one into it, then delete your old notebook.
 

2. From online backup

If you deleted your notebook and soon noticed that some of your notes are missing, you can restore them from backup.
 
I never delete backups, because the cost of disk space is much lower than the cost of our time.
 

3. From downloaded backup

If you deleted your online backup, but downloaded it to your computer, you can still upload and restore it.