Microsoft says on the official Xbox site that the issue was a temporary one which has since been resolved. Those who are yet to see their Gamerscore restored are advised to log out and back in.
ORIGINAL STORY: Microsoft is investigating a bug that is wipingXbox Live users' Gamerscores when they unlock an achievement.
Forums - including Microsoft's own - and the Xbox Support Twitter account are currently awash with complaints from users who have unlocked a new achievement to find their Gamerscore being reset to zero.
On Twitter, the Xbox support team has suggested users try logging out, clearing the console's cache and recovering their Live profiles, but it appears doing so does not correct the issue in every case.

http://www.edge-online.com/news/xbox...d-update-fixed