Earlier today Xbox Live users were experiencing problems with the gaming history in their Xbox live profiles. A mess of crap was happening including an entire loss of gaming history and resetting precious GamerScores to zero.
It seems that loading up a game or earning an achievement would cause this loss of data for a majority of its users, resulting in achievement history being replaced and reset, games unlocking all of its achievements at once upon loading, loss of purchase history and profile data (such as the self-edited biographies or Avatar’s clothes).
The situation has since been resolved, but not without a few drawbacks, it seems that any achievements earned during this time cannot be re-unlocked if they were removed during the fix. Users affected by the data drop were suggested to recover their account details.
If you have not been logged into a Live service between 1PM UTC and 7PM UTC (Between 6AM and 12PM Pacific Daylight Time), then it’s highly unlikely you will have been affected by the data loss and can continue using the service as normal. For those who have been affected, contact Xbox Customer Support if recovering the account does not remedy the situation.
It is unclear why this happened, the gossip will state that Microsoft were the victims of an attack, but minor syncing issues noticed a couple of days ago may have something to do with it. I doubt it has been a hack for monetary sake as no one has reported any monetary loss from this hiccup.
As of right now the service is running as normal again.