How can I recover accidentally deleted folders from Notes?

Am no expert in this area, yet your request deserves some response.

Wondering if Apple support's online articles can help you:

What to do: If you're missing notes on your iPhone, iPad, or iPod touch - Apple Support

or

If you use iCloud, see how to: Delete and recover notes on iCloud.com - Apple Support

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Else if you have iPhone back-ups, you may be able to restore the folders that way:

(NB: You may first need to make a back-up of your current data, to keep it safe)

: Locate backups of your iPhone, iPad, and iPod touch - Apple Support

and

See how to: Restore all content to iPhone from a backup - Apple Support

All the best :-)