Trying to Upgrade to 2.8.4 has my SITE SHUT DOWN

My site is shut down.

I tried to upgrade to 2.8.4 and it won’t let anyone onsite now.

1. We got hacked a few days ago.
2. A hidden administrator appeared and we started getting a password box which popped up whenever we posted. Then it appeared on our site. All of this occurred within about an hour.
3. I found and removed the hidden administrator at MySQL. I then downloaded 2.8.4 and unpacked it.
4. I transferred the files to my site via FTP this afternoon and whenever you type in “https://deathby1000papercuts.com” you now get the following error message:
“Fatal error: Class wp_widget_pages: Cannot inherit from undefined class wp_widget in /homepages/4/d226805084/htdocs/wp-includes/default-widgets.php on line 15”

(I’ve been away for the last 8 months and was going to put off upgrading until next week, but the appearance of the hacker forced me back early. I’m not up to speed yet and can’t remember if I forgot something simple. My config files are the same and still have all the relevant info on them that points WP to the database.)

Help! Anyone have an idea how to correct all of this?

THANKS in advance.

mondo