Upgrading IOS 5, iTunes and iPhone 4 cause the loss of data?

Can someone guide me to restore some important data that I lost recently while performing an upgrade to IOS 5. I also updated iTunes and iphone4 on my computer. After this upgrade all the calendar notes that I had are missing. I don’t know whether these notes were safely moved to some other place. Everything else is functioning properly including Hotmail. Please help me out to restore the data.
