There are several things that you can do to try and fix this issue. Please follow these steps:

1. If your Mac is using OneNote 16.25 or earlier, download and install the latest version of OneNote.


2. Log out and log back into your account.


3. Restart your Mac (by clicking the Apple symbol in the upper left corner, then clicking Restart).


4. Verify that the default font and font size are set correctly in the settings.


5. Bring up the ribbon (by selecting View > Show Ribbon).


6. Select the font and font size from the drop-down menu in the ribbon.