One Student Does Not Have Content Library in her class notebook

If her name isn't appearing in the Members list but she already has a personal section, that does point to an account or membership sync issue rather than anything she changed herself.

I've seen this happen when a student's Microsoft 365 account or class roster membership doesn't sync correctly. Usually, removing and re-adding the student to the class (or having IT repair the notebook membership) resolves it. It's also worth checking that she's signed in with the correct school account, as using a different account can cause sections to appear missing.

I think involving your IT department is the right next step, since they can verify the roster and permissions from the admin side.

The student probably did not disable the Content Library herself. That section uses special read-only permissions, and if those permissions are interrupted, OneNote can hide the library from affected students while the content remains intact. As the notebook owner, open the Class Notebook permissions repair page and run Fix Permissions for this notebook. Confirm that the Content Library section group is still at the notebook’s top level and has not been moved through ordinary OneNote operations. After the repair completes, ask the student to close the notebook fully and reopen it in OneNote for the web before testing the desktop app. Also verify that she remains a member of the correct class. For a notebook created in Microsoft Teams, add or remove students through the class team rather than the Class Notebook Wizard. Avoid sharing the Content Library manually, because that can bypass the notebook’s managed permission structure.

Hi anna2teach​ 

Thanks for sharing this. if only one student is affected, it is unlikely that the Content Library was accidently turned off. i would first verify the student has been added correctly to the class, then ask them to reopen the notebook or try accessing it in OneNote on the web. if other students can access the Content Library, it may be a synchronization issue specific to that student's notebook.