Error upgrading WP 3.1

Hi, yesterday i tried to update my wordpress site, something happened and the installation ended with an error. I tried to enter to upgrade again and it doesnt work, i tried to upgrade manually and a similar error its given. I had to upload the backup again, but i cant get to the inside of my site. this is the error:

Fatal error: Call to undefined function is_network_admin() in /home/pr151182/public_html/wp-admin/admin-header.php on line 16

Help me please, i dont know what to do.