Installed WordPress 3.1 and it gave me a error message

“Fatal error: Call to undefined function” is always a sign of an incomplete upgrade, where not all the files got upgraded.

Do a manual upgrade.

Hello,
That error is showing up I cannot login into my admin page.

When I go to https://www.fashionwithoutguilt.com/wp-admin I get this error:

Fatal error: Call to undefined function is_network_admin() in /home/content/91/3658291/html/wp-admin/includes/template.php on line 2097

Try and do a manual upgrade by using FTP or whatever file management application your host provides.

Hello,
I say this error at the top of the linked page you gave me do you think I lost all my post and everything?

This is the message I am talking about:
The upgrade process will affect all files and folders included in the main WordPress installation. This includes all the core files used to run WordPress, two plugins (Akismet and Hello Dolly) and one theme (Twenty Ten). If you have made any modifications to those files, your changes will be lost.

Your posts are fine. You need to do a manual upgrade. Please read the instructions linked to above.

If you don’t ftp the wp-content folder over, you won’t lose any changes you may have made to the twenty ten theme.