Fatal Error message from WP Update attempt

I tried doing the automatic update on my site, http://www.PaddleGrandPrix.com, and it failed several times, giving me the “try again” message. On the final attempt, whatever went wrong now results in the following error message every time I try to log into my dashboard:
Fatal error: Call to undefined function is_network_admin() in /home/paddlegr/public_html/wp-admin/admin-header.php on line 16
Can anyone help advise me how to get this patched up and working again? Thanks!