Please check Archive. Change sort option from ‘Archived’ to ‘All’ and see if there are missing notes. If you cannot find notes in Archive, check Trash can. You can go to Archive and Trash can by tapping the ≡ button on the bottom bar. Also search notes using the ‘Search’ button.
If you cannot find the note with the above methods, restore notes using Device backup. Your notes would be automatically backed up to your device, unless you have turned off Auto backup in Settings.