Lost a folder in Notes

Hello Jwfone,

Notes that were deleted should be able to be retrieved.

Delete and recover notes on iCloud.com - Apple Support

If you are not able to find your notes via Apple systems then this may be an issue were the notes were in a folder being populated by your email account. If you have or have had any other email account set up then the notes may have been syncing through that account instead of iCloud. The notes app is capable of including any email account added to the phone, that has an ability to sync notes (most do), as an option to save notes too.