Using Classic Outlook on Windows for personal email, calendar, and contact management
That is a common issue with iCloud.
When iCloud disappears from Outlook, make sure you have the latest version of iCloud installed (it gets regular updates). Then use the iCloud Control Panel app to sign out of iCloud and then sign back in again and re-enable the Mail, Contacts, Calendars and Tasks sync again.
A similar scenario is discussed here; Contacts and Calendar from iCloud missing or not synching after upgrading to Windows 10 or Outlook 2016