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 find Archive and Trash can in the side menu. Also search notes using the ‘Search’ button.

If you cannot find the note with the above methods, restore notes from SD card. Your notes would be automatically backed up to the SD card, unless you have turned off Auto backup in Settings.