My past calendar is missing... Any ideas?

Welcome to the Apple community.

I'm not sure if this is what you mean, but:

Settings > Mail, Contacts, Calendars, scroll down to the Calendars section and change the sync option to something other than 'Events 1 month back'.

This solved my problem too!

Thanks, but it was already set at "all events." I tried other time periods as an experiment, but still no calendar entries.