If you can't find some of your OneNote 2016 notes, they may not be gone for good. OneNote keeps a limited set of automatic backups by default. Where you look for them depends on where your notes are stored.
Search for backups on your computer
If you’re working on notes that were stored on your computer, do the following:
Open the notebook where you lost recent notes.
Choose File > Info > Open Backups.
In the Open Backup dialog box, note the folder names that appear. Each folder represents a OneNote notebook that’s stored on your computer.
Double-click the notebook folder you want to open.
Select the notebook section backup file you want to open, and then choose Open.
If your deleted notes are included in the backup sections or pages, right-click the section tab or individual page tab that you'd like to restore, and then use the Move or Copy command to put the notes back in their original location.
Important: To avoid any confusion between backup notes and your normal notes, be sure to close the backup section when you’re done inspecting it. In the Open Sections view, right-click the section tab of your backup, and then choose Close.
Search for backups on a notebook in OneDrive
If you’re working on notes that were stored in OneDrive, do the following:
Open the shared notebook where you lost recent notes.
Choose History > Notebook Recycle Bin > Notebook Recycle Bin.
If your deleted notes are found there, right-click the page tab that you want to restore, and then use the Move or Copy command to put the notes back in their original location.
Any notes that were deleted during the past 60 days should appear in the Notebook Recycle Bin, unless the Notebook Recycle Bin was emptied during that time.
Important: To avoid any confusion between backup notes and your normal notes, be sure to close the Recycle Bin when you’re done inspecting any available backups. Click the green, upward-pointing arrow next to the Deleted Notes section tab to return to the previous level.
To help protect yourself from future data loss, you can customize the automatic backup settings in OneNote.