You should always back up your notebooks, just like the rest of your computer’s files and data. Hard drive crashes and human error can happen to anyone, and if it happens to you, your notes will be gone forever. If you regularly back up your notebooks, you can restore them whenever you need to.
OneNote can keep manual and even automatic backups of all of your work, but that's not currently possible in OneNote Online. For detailed steps on how to do this in the OneNote 2016 desktop app, see Back up notes and Restore notes from a backup.