Fatal Error Message during install

I need your help. I have a blog that I occasionally post to. I am not an expert by far so please try to keep your responses simple (and not snarky)!

I recv’d. an email from my host that there was a virus file uploaded to my site.:
“This is an urgent message concerning your __ account. Our anti-virus scanner has reported that a malicious file has been uploaded to your webspace.
Name of the file: ~/wp-content/themes/bota/page-templates/hash_file.php”

I logged in and saw almost 700 spam comments. I starting deleting them and then everything froze and I got a message that the comments were in moderation.

I clicked on the upgrade link and a short time later got this message: Fatal error: Call to undefined function wp_installing() in /homepages/13/d231473180/htdocs/wp-includes/functions.php on line 1359

The entire site is down. I logged in using ftp and located the virus file and deleted it. Nothing changed. No, I did not backup the site before trying to upgrade. Any help would be appreciated! Thanks!!!

Celestina