Here is a list of things you can do if you are unable to view all of your saved notebooks in OneNote:

1. Check the sidebar in OneNote to make sure all your notebooks are listed
2. Check your iCloud Drive to make sure all the notebooks are listed and properly synced
3. If the notebooks are not listed in either of the two locations, try restarting your device
4. Reset your device’s OneNote app settings by going into your device settings and resetting the app.
5. If all else fails, reinstall OneNote