This same issue happened to me after I converted a previous account into an Xbox Live account. I wanted to check the Event Calendar, so I went to dashboard and did the conversion process. When I finished, I restarted the game to look at the calendar.
All Unlocked Character, My level 17 rank, weapon skins, medals, and ribbons were gone. I however retained my Campaign data, which is on the very last level (which I beat, but the game no longer acknowledges).
I tried typical things like sign out and back in, restart game, restart Xbox. I have not messed with any of the data cache or the hard drive itself because it isn't mine to poke around in.
EDIT UPDATE: Also I'd like to note I still have all achievements for the things that I did do. (Beat the game, reach levels, gain collectibles, etc.)