unable to login – fatal error message

So starting this morning, every time I’ve tried to access my blog’s back end I get this error message, which means nothing to me:

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

The blog itself is still up and functioning, but I can’t get into it. I’ve also been having utterly no luck for the past 3 weeks when attempting to upgrade to the 3.1 version of WP, and I wonder if this is in any way related to my current problem. It seems all kinds of back end things have suddenly become buggy.

Please help! I have no clue what’s going on, but it’s driving me crazy, and I absolutely need to get into my blog’s admin pages asap.

My blog is located at: https://www.nheilke.com/blog

Thanks!