Something weird just happened, never realised when it actually happened.
All my categories seem to be missing. When I go to edit all my posts, it lists all my posts as "Uncategorized" and when I click on the Categories page, there are no categories listed.
However, when I try to add a new category under the same name as one of my old categories, it tells me that that name already exists. Also, on my Dashboard under the "At a Glance" section, it says that i have 6 categories, which is the correct number.
What the hell is going on here?!
EDIT: I just realised ALL my tags are missing from my posts as well?! WHAT?!
EDIT 2: Is it possible that this happened because I installed an older version of the plugin Google Sitemaps Generator which wasn't completely tested for wordpress 2.7?