UPDATE: Microsoft is saying they've fixed the issue and you can restore your gamerscore by signing out and choosing to download your profile again (no need to delete it first). it worked for me - for now. The issue wasn't Mass Effect 3 specific.
I fired up Mass Effect 3 just now and as the game loaded I got a message saying I last played it on a different console and it would now load my own save data. I figured it was some kind of glitch in the game and just hit ok.
When I started playing Galaxy at War I hit one guy with a biotic power and unlocked "Sky High." Remembering I had already unlocked that achievement I went to my profile to check and sure enough my gamerscore was down to 15, with Sky High the only achievement at all unlocked!
Any idea what happened? How can I fix this?
I've had my gamertag since original Xbox and have tons of achievements and completed games. This is really frustrating!