You could possibly retrieve you lost data by simply syncing your phone with your computer by plugging it in via USB port. Hopefully you at least used your computer once with the phone which stores and saves info. As to what could have caused it, not to sure. Only contacts and calendar events? Or were apps affected to?
There are two ways for you to restore iPhone data: directly scan your phone and extract your iTunes backup. These two ways let you restore contacts, SMS, photos and more. Read more: http://goo.gl/PvUo6e.