My categories have vanished.

After creating a post earlier this evening, I went to select a category but all of them were missing. I checked my list of categories under the Manage page, and not only were they gone, but when I went to replace them, I was not permitted because those categories “already existed.”
All of my posts have reverted to “uncategorized” as well.

I believe the same thing may be happening to a friend of mine, because when I went to look at her website, her posts had also reverted to “uncategorized.” Is this happening to anyone else? And does anyone know a way to bring my categories back?