Hi Clare
Greetings
Thank you for posting in Microsoft Community.
Please try to open Outlook in safe mode and try to change the font to see if the issue still persists. To open Outlook in safe mode kindly refer to Open Outlook in safe mode (microsoft.com).
If your can change your font when in Safe mode, then please try to disable add-ins, instructions on how to do so are in the following article I can’t start Microsoft Outlook or receive the error “Cannot start Microsoft Office Outlook. Cannot open the Outlook Window”
Lastly if opening Outlook in safe mode does not help please try to run an Office Repair on Outlook, for instructions on how to do see please see Repair an Office application (microsoft.com)
NB: Before you carry out a Repair on an office application please make sure you back up your office for data loss prevention.
Looking forward to hearing from you.
Regards,
Momo