Custom Wordpress Site Gone Offline - **FAST JOB**

I have a wordpress website that scrapes data from a website with a proxy. The scraped information is then stored in a database. It then uploads the scraped information once a day, and also places affiliate links into the pages. Yesterday the site went offline, the hosting company has shown me nobody has FTP'd into it, and there is no traces of malware or viruses. The previous owner of the site can't be reached, and I'm now left with a website which has mysteriously gone offline. The first error I received was a database error. After getting the site backed up, the error now reads - Fatal error: Call to undefined function is_multisite() in /home/livefoot/public_html/wp-includes/[login to view URL] on line 20 The hosting company unfortunately backed up the site with a copy of the downed site. I have the original files here, but after re-uploading them onto another server they still won't work. I need an expert to go onto the server and fix this inside the next 24hours. No new guys, no chancers, professionals only. Thanks, Gilesy