Wordpress Call to undefined function is_network_admin() in wp-admin/admin-header.php

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?
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birwinAsked:
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JF0Connect With a Mentor Commented:
Did you also have a backup of the database before you tried the upgrade?  If so, you need to use that backup as well as the site backup.
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birwinAuthor Commented:
I restored the database, and get the same error.
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jeremyjared74Connect With a Mentor Commented:
Have you tried removing all themes/plug-ins except Twenty-Ten?

Start there and let me know how it turns out.

You may also want to try downloading Wordpress and installing it manually. You can drag and drop the entire contents within the Wordpress folder and replace the existing files. WP doesn't come with a wp-config.php file (it has wp-config-sample.php), so nothing will be overwritten. If there was any packet loss during the download it could work, but have be "buggy". I have had that happen before.
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gwkgConnect With a Mentor Commented:
Try renaming the plugins folder.  If it works after that then it's a plugin issue.  Add them back one by one until you find the culprit.
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birwinAuthor Commented:
Thanks for your help. I gave up and hired a freelancer with Wordpress experience, and he fixed it.
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