Has the preference under Settings - Mail, Contacts, Calendar - scroll down to Calendar and then 'sync' been changed? Mine is set for 'Events 6 months back' You can change the settings to a different time period.
I've just checked my calendar on the iPad and everything to the end of March has disappeared apart from yearly repeats. I haven't done anything to it to cause this.
Has anyone got any idea how I can get the data back?