Welcome to Android Central! Are you missing all of your contacts, or just one? Were they saved to your Google account, or only to the local Phone account? Go to contacts.google.com on your computer browser -- are they there?
How did you update the phone? Was it an official over-the-air system update, or did you download the file from a website and manually install it? An over-the-air update shouldn't cause a factory reset, but certain manual methods of installing firmware might do that.