All my past events disappeared. Why?

This is likely happening due to the fact that you have setup your device to not fetch old events. To review this setting, please follow the steps below:

1. Go to iPhone Settings
2. Go to Mail, Contacts, Calendars
3. Scroll down to the Calendars section. Within that section, tap the "Sync" cell.
4. Please review the sync settings. If you want ALL past events to be synced, tap "All Events".