[Ixia]

My OneNote has stopped backing up onto OneDrive, can't see anything in settings to enable this again?

[FAHMI BASSEM]

OneNote does sync automatically whenever you open the app, for the first time you need to tap on each notebook and wait tell the sections show up

[hotphil]

Backing up or syncing? Where are your Notebook files themselves actually stored?

[Ixia]

They used to be stored under documents on Onedrive and I liked that as I could access them from any computer. but since I've had my phone mended no OneNote files have been backed up there.

[-8-]

So you mean that Onenote on your pc still works and syncs with the notebook on your Onedrive but your phone doesn't?
If so, first try a manual sync: tap More (...), then synchronize.
Also check if you're in the right notebook: on top of Onenote keep tapping back until you're at the notebooks and make sure it's the same one as on Onedrive and not a local notebook.