Fatal Error Suddenly Appears on Website

My blog is https://www.afantasticnightmare.com.

Today I logged in, wrote a couple of posts and saved them as drafts. Then, I upgraded my WordPress blog to 3.0 and logged out or closed out of the window.

Now, I return to the login page and this is the message that appears:

Fatal error: Call to a member function add_rewrite_tag() on a non-object in /home/content/79/5765779/html/wp-includes/taxonomy.php on line 289

What does this mean and what should I do?

Someone help!!!