I have the same problem. It has happened twice in the last week. Nobody here has actually given a solution to the problem.
Here are the particulars:
-iPhone 4S
-iOS version 7.0.4
-I use Game Center, but I have never loaded Clash of Clans. I do have Candy Crush and Subway Surfers, both seem to have adds that link to CofC. I don't know if they are made by the same company.
-I don't use Do Not Disturb.
-I have a .mac email account that I use for iCloud credentials.
-Seems to only happen when I am home and the iPhone is synching with WiFi.
-The problem begins with a dialog window on the phone that requests that I re-enter my iCloud credentials.
-Within a few minutes/hours, all of the iCloud information is gone (contacts, calendars)
-Email seems to still work for the iCloud and all other email accounts (gmail, hotmail, etc.)
-The only way to get back the information is to re-boot the phone.
-Prior to the dialog window, nothing out of the ordinary has taken place.
-I am a little OCD, so I permanently close all running programs except for Mail, Messages, and Phone.
-Programs running prior to the problem could be many but typically I use Waze while driving home, and I would have closed it after I pull in to the garage. (My iPhone gets WiFi in the garage)
-The biggest drag is that when the contacts get cleared, any personal ringtones, or other identifiers get permanently cleared and have to be reassigned. That is the most annoying part.
What else does anyone need to know? It seems to be a thing, so when (and I'm sure it will repeat) it happens again I will throroughly check all suggestions.
I know how to get my data back. I would like to know how to prevent it from ever happening again.