Xbox Live gamers have been coming out of the woodwork this morning with stories about playing games today and subsequently having their Gamerscores drop to zero and their histories erased. Based on the nature of the complaints, it seems that the problem occurs when players earn a new achievement in a game. Based on multiple accounts, the glitch doesn't seem to be linked to specific games.
Microsoft has posted on their support page that they're working on a solution and that players' scores have not been permanently erased. If your gamerscore has "zeroed out", you can recover your score by recovering your gamertag via Xbox Live.
Multiple commenters from the Xbox forums say they've been told that the Xbox servers are being updated, causing the glitch. If that's the case, the problem should be fixed later today.
We'll update you on the situation when more information becomes available.
UPDATE: Microsoft says that they have now fixed the achievement point issue. If you've lost your Gamerscore, logging out of Xbox Live and logging back in should fix your account.
If that doesn't work, Microsoft says to recover your profile via Xbox Live. If you recover your profile, you will lose any achievements you earned since the account malfunctioned.