Downgrade question

After being unable to get my upgrade to work, I replaced all the new files upgraded today with the old files. Everything seems to be there and working for the viewer.
But, I can’t log in, I get this –
Fatal error: Call to undefined function: wp_admin_css() in /home/valmg/public_html/wp-admin/admin-header.php on line 20

Will someone please let me know what I need to do to be able to log in again?
Thanks.