Based on your description, you're not able to type anything into your existing notes after updating OneNote for Windows 10. Given this situation, here are some troubleshooting steps that you can try so we can properly isolate the issue:
NOTE: Before proceeding, please make sure that all your notebooks are synced to the cloud to avoid possible data loss.
1. Sign out your account from OneNote and sign in again.
2. Resync the notebook by right-clicking on the notebook name then select Sync > Sync This Notebook.
3. Close the problematic notebook then open again. See if the issue will persist.
4. Open your notebook in OneNote online then click on the 'Open in OneNote'.
If the issue persists after trying all the steps above, we would like to confirm the following information:
1. What happens when you try to type on the existing pages? Are you receiving any error messages?
2. Have you received any sync error after updating?
3. Are you the owner of the notebook or it's a shared notebook?
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