The same happened to me: lost all my info from my calendar after the upgrade. But I just got them back! Maybe it will work for you too: go to Settings/Mail, Contacts, Calendar. In the Calendar section click "Default Calendar" and put a checkmark next to "Home". Get out of there and check your Calendar. I got all my data back! Then I changed the "default calendar" setting back to "Calendar" and I still had all my lost info. I hope it helps.