Help! Fatal error: Call to undefined function is_user_logged_in()

Try manually resetting your plugins. If that resolves the issue, reactivate each one individually until you find the cause.

If that does not resolve the issue, access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This which will force the Default theme (WordPress 1.5 – 2.9.2) or the Twenty Ten theme (WordPress 3.0 and higher) to activate and hopefully rule-out a theme-specific issue.

You’re welcome!