I am using the stock calendar app, synced only to one account (my Gmail account) and after JB all my calendar entries were gone! (Contacts were fine) Logged on to Gmail with my PC, and everything still there. Somehow after JB my calendar app lost the link to my Gmail account. From the (empty) calendar app, I selected Menu, Calendars to display, and noticed my Gmail account wasn't listed. At the bottom of that screen there is a button that says "Calendars to sync". From that I was able to re-establish the link, then sync, and I was a happy camper.