Go here: http://www.facebook.com/help/identify.php?show_form=forgot_login
The form won't restore your account (or at least it didn't for mine, though it recognized things which made me feel better). BUT IT WILL SEND AN EMAIL TO FACEBOOK AND THEY'LL CONTACT YOU. So that's the first step, I think.
To add on: I did contact them this morning, haven't heard back. I found a form that has you enter a bunch of information to locate your account - you enter in the name of one of your friends, your current town, and the URL of your account (if you know it). I haven't heard back yet - I'll link it so everyone can email. Hopefully that'll make them realize it's a widespread problem. It once took me two months and 8 to hear back from them, so I think it's important everyone let them know.
Same thing happened to me. I deactivated at 9:37 last night, and when I went to re-activate it three hours later I got the same error message. I filled out all the information in the request, but it sounds like there's a glitch on Facebook's end.
I do deactivate and reactivate once or twice a day - maybe they're trying to prevent people from doing that? Either way, this question is all over Yahoo Answers right now, so I'd bet it's a problem only Facebook can solve. And hopefully they'll do so quickly :-)
Also, there's a thread on Facebook about people having the same problem.
But unlike ALL the other threads, this one doesn't have the option to answer. The last answer was posted an hour ago, and nothing since then. I hope that means they know about it and are fixing it, and not that they're trying to keep people from talking about the problem.