Can't login into wp-admin – Undefined Function?!

I suddenly can’t login into wp-admin, this is the error message I get in the browser:

“Fatal error: Call to undefined function is_network_admin() in /customers/thewildpony.com/thewildpony.com/httpd.www/wp-admin/includes/menu.php on line 10”
________

I found the file – and this is the text (line 10-15):

if ( is_network_admin() )
do_action(‘_network_admin_menu’);
elseif ( is_user_admin() )
do_action(‘_user_admin_menu’);
else
do_action(‘_admin_menu’);

_______

Can you please help me and tell me what to do next? I’m so afraid to loose my 100 blogposts – and followers!

The error showed up after trying to update from 3.0.5. to 3.1. I didn’t make a backup until after the errormessage – but I hope it’s all saved anyway. It looks right at the domain it self – I just can’t login to do any updates.

I hope to get some help in here.
THANK YOU!

Sandra

[I’m sorry to post this one in two diff. forums – didn’t know which one to choose]