I tried to upgrade to Wordpress 3.1. It crashed and rendered the program useless.
I had a site backup from the previous day. I renamed the old wordpress folder to wordpress_OLD and recopied the previous day's wordpress directory to my server. The main site works fine, and I can post articles, after changing a few file permissions.
But when I try to access the admin panel, I get "Fatal error: Call to undefined function is_network_admin() in /home/mysite/public_html/wordpress/wp-admin/admin-header.php on line 16
Any idea how to fix this?