OneNote automatically saves your notes while you work. If your OneNote is set up to be stored locally on your PC and not on a OneDrive, you should regularly back up your notebooks in the same way as the rest of your files and data on your PC. It can happen to anyone that the hard drive breaks or the PC gets stolen, and if it happens to you, your notes will be gone forever.
Open OneNote (not the OneNote app).
Select the “Files” tab.
Select "Save and Backup".
On the right side of the window, select "Backup folder" and click "Edit".
Then select a new destination folder for your backup.
Once you select the destination folder, click the "Select" button.
Then, click on the "Back up changed files now" button and then on "Back up all notebooks now".
After backup, you will receive the message "Backup complete".
Finish by clicking OK at the bottom of the window.
You have now backed up your notes and, for your safety, check that a copy of your OneNote files is now in your backup destination folder.
After backing up Onenote, you can restore your notes if / when you need them.
Open OneNote and select "Files" and then "Information". Select "Open Backups".
Browse / click to the folder where you saved your backup. Click to "Notebook name". Select the section(s) you want to recreate, and then click the Open button.
At the top of "Open Sections", the selected sections you just opened from the backup files will now appear.
Now your notes are restored.