If you have taken auto-backup of the notes before then you can restore them now during a fresh new app installation OR do this manually from the existing app.
Currently, we are supporting auto-backup restoration (during app installation) only from Dropbox or Google Drive. You can however import the backup files manually into the app as said above.
To import the notebooks from the backup location, open Noteshelf -> Tap on the “+” icon at the right side bottom corner -> “Import Document” -> Select the “.nsa” file from Google Drive (if your backup location is Dropbox/OneDrive, then download the files first and select them from the downloaded location).
Note: If you do not have a prior backup OR deleted/reinstalled the app without taking a backup then unfortunately, there is no other way to get back the data now as we do not store the data on our servers, for privacy & security reasons.