UPDATE: Microsoft has issued a statement on this morning's shenanigans, and claims everything should be fixed in a few hours.
"We are aware that some Xbox LIVE members are having problems with their Gamerscores. Affected users should log out and back in to restore their scores. While this should fix the issue, updated Gamerscores may not be reflected on Xbox.com for a couple of hours. We’re working on this and expect it to be fully resolved soon.”
As for achievements earned during the glitched period, Microsoft told me the company is working to have everything restored "shortly."
This specific issue has reportedly been addressed, according to Xbox Support, but there are lingering problems.
“I want to let you know that we're aware of an issue with achievements, and that we're doing everything we can to resolve it ASAP,” said Xbox Support representative Mister Fitzer in a thread.
If you’re one of the people with a suddenly missing gamerscore, and thus missing achievements, the first step is to sign out of Xbox Live and sign back in. If that doesn’t work, try re-downloading your profile. Hopefully that works, but any achievements you earned during this weird period have to be earned all over again. That's unfortunate.
For now, however, it’s probably best to stay off Xbox Live.
Make sure to report any of your own issues in the comments.