Dear Kornelija Vitkutė,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
We are sorry to hear that you’re having trouble with adding back content library. I deeply understand the inconvenience caused and apologize for it.
We appreciate your understanding that sometimes the initial response may not resolve the problem immediately. However, we can work together to narrow down and resolve the situation. So, for further troubleshooting, could you please provide the information below:
- May I know, if the issue still persists if you open the Class Notebook from OneNote.com.
- Could you please let me know if you tried to run the diagnostics from this article: Fix Class Notebook when Welcome, Collaboration Space, or Content Library are missing - Microsoft Support?
- Could you please provide a full screen screenshot of the issue? You can upload the screenshot using this feature:
Once again, I apologize for the inconvenience this has caused. We appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Ralph Chawatama | Microsoft Community Moderator