I was playing Skyrim a short time ago, turned my console off for about 15 minutes, then came back and reloaded the game. As the game was loading, a message popped up saying something about having played the game recently on another console (I'm not sure of the exact message, as I spammed the A button thinking it was going to ask me to select a storage device). I exited the game and checked that I everything was in order, but my Skyrim achievements had been reset from 22/50 to 0/50 and the Skyrim gamerscore from 360/1000 to 0/1000. My actual gamertag gamescore hasn't changed.
If I examine the Skyrim achievements panel, all 22 of the old achievements are highlighted as having been unlocked, even though the total says zero.
If I check my profile here on xbox.com, it continues to report the old achievement totals (22/50 and 360/1000), but my xbox still says 0/50 and 0/1000.