Restore failed — call to undefined function

I tried upgrading to 2.2, but received errors I couldn’t resolve. So I tried restoring back to my old version. Before upgrading I backed up my database, and I restored it using these instructions: https://www.tamba2.org.uk/wordpress/restore/

There isn’t a whole lot of info on how to restore a blog after restoring the database. I deleted the appropriate files and reuploaded the 2.1.2 wordpress files.

However, when I go to my blog, I see this: Fatal error: Call to undefined function: wp() in /home/idrathe1/public_html/wp-blog-header.php on line 16

I’m at my wits end, having spend the last 2.5 hrs with WordPress. Can anyone provide help to me? Thanks. My blog is at https://idratherbewriting.com