It's not possible to recover those notes unless the Notepad application had a function to upload them to a cloud storage service (Gdrive, Dropbox, Owncloud, etc). If the files are stored on the phone, then the thief has access to all the data on that phone. I would like to recommend contacting your carrier to block the SIM card and eventually issue a block on the phone's IMEI. That's the only thing you can do.