You deleted your calendars on your last backup. Well a backup is one thing, a sync is something else, and then however you deleted the calendar could be something else. How were you backing up? What did you do that deleted the calendar?
When performing a sync, the phone does a backup first, then the sync. Was the calendar on the computer or on the phone? Were you using iCloud perhaps? Were you using a supported application on the computer to sync your calendar?