Unable to select from list of accounts

When I compose an email, my FROM button drop-down is completely blank. 

None of the many email accounts I have configured show up.

Even the 'from' line (to the right of the FROM button) is blank.

Does anyone know why this could be happening?  It's happening on multiple computers (Windows 10) in Outlook version 2303 build 16227.20280.

Note that I can still end the email but I can't select which account it's being sent from.

Thanks in advance for any help!