Fatal Error After WP 3.0 Upgrade

After upgrading to WordPress 3.0 from 2.9.2, I get this weird message as follows:

Fatal error: Call to undefined function use_codepress() in /home/e2pcom/public_html/wp-content/themes/thesis_17/lib/admin/admin.php on line 42

This message displays after I click on the Custom File Editor, thus, I do not have access to custom.css, layout.css and custom functions.php to make revisions to the code.

Please help.

Yours,

Jae