iPhone 5 upgrade to iOS 6.1 deleted all my calender events from Outlook

I upgraded my iPhone 5 to iOS 6.1 (from 6.0).

 

Also updated my iTune to 11.0.1.

 

After upgrading both my iPhone5 and iTunes the very next sync deleted all my events from Exchange calender. Why did that happen?

 

And how can I recover those events. 400+ calender events were deleted.

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