How to restore my calendar on my iPhone

Are you syncing the Calendar to iCloud? Try going to System Preferences/iCloud and stop syncing by unchecking the box. Wait a couple of minutes, then check syncing. Allow time for the sync to complete.

Contacts, Calendars, or Reminders - Get help using iCloud

Last resort  - have you tried signing out of iCloud and then sign back in?

If you aren't syncing the Calendar to iCloud, restore from your backup.

Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore.

Backup - Restore your device from an iCloud or iTunes backup