UPDATE 2: Now Microsoft is advising those affected to delete and recover their profiles.
UPDATE 1: Microsoft has suggested those affected by today's Gamerscore wipe sign out and then back in to their profile to resolve the issue.
ORIGINAL STORY: Eurogamer has been alerted to widespread reports of Xbox 360 Achievements being completely wiped.
Numerous readers have written in with personal accounts of their score and Achievement tally disappearing.
The Xbox.com forums are awash with complaints of a similar nature.
The Xbox Support Twitter account is telling worried gamers that Microsoft is "looking into this right now".
Eurogamer has contacted Microsoft for comment.
What follows are accounts from Eurogamer readers who alerted us to the issue.
"Turned on my Xbox this morning and booted Mass Effect 2. Got an Achievement, hit the Guide button to check out what. And then I discovered that my Gamerscore had been reset to zero," wrote Mark (surname withheld). "I've been subscribed to Live for almost four years, racked up a humble 7000 Ggamerpoints over 20-odd titles.
"All of it was gone," he added, "bar the 15G Achievement I'd just unlocked. All my history of other Achievements in other games had been completely wiped, the whole history."
Mark said he'd phoned Xbox Support who were "aware" of the problem and had had "40-odd" other calls of a similar nature.
"No idea what caused it," Mark closed, "but naturally I'm gutted. And I'd imagine a lot of other people will be too. "