"A bug in the Xbox Live service has lead to problems with leader boards and stats for Xbox 360 titles, but the issue has been identified and many of the problems have now been resolved.
Many Xbox 360 gamers were greeted in the New Year with wiped Xbox Live stats, and thoughts that those once in a lifetime achievements would be lost forever. The bug in the Xbox Live service has now been found and in many cases things are back as they were. According to a post on Major Nelson's blog, Geometry Wars: Retro Evolved is the only title that still has leader board problems.
"The issue is now fixed, and stats have been restored, with the exception of Geometry Wars: Evolved. The team continues to work on that title's leaderboards," posted The Major." [link]