Site hacked and then down, after Upgraded

Hi folks

We had a client’s site hacked recently, injected with spam, it seems.
When the client logged in to check on it, he clicked to upgrade when he saw the message prompting him to do so.

During update the site went down and this error went up:
“The file ‘wp-config.php’ already exists. If you need to reset any of the configuration items in this file, please delete it first. You may try installing now.”

I cant get into the admin section at all, and the public site shows that message as well.
It seems to want to install wordpress from scratch and I am worried about loosing my theme and content from the site.

Via FTP, i have also renamed the plugins folder, but nothing came of that.

Does anyone know how to fix this or why it happened?

the site’s original version is unknown – but was installed about 4 months ago. I have access to the MySql db, so if its written there anywhere, i can check.

Thanks!