After upgrading to iOS 5, I lost my all calendars, contacts, and message data Today I upgraded my iPhone 4 to the newest iOS. In the process steps it asked something like start from new iOS, or start from your back-up in iTunes, I selected the first one and then noticed there are no contacts, nor messages!

My iTunes had a problem and I thought I had backed up the files in my iTunes but I checked ~/Library/Application Support/MobileSync/Backup
This is what you do:


Go to settings
Click on Mail, Contacts and Calendars
Scroll down and go to Calendars
Open the Sync screen 
Check the box which says All Events
In about five minutes time, go back to your calendar which should have all of your previous events restored.


Happy days.