It’s the worst nightmare of every webmaster, your site gets hacked. That’s really bad for you as a webmaster but also for your search engine rankings. But Google can help you get back in the high search engine results pages again.
In early may there was some kind of backdoor in the shared server of my hosting company. Because of this backdoor a javascript changed all the index pages of every website on the server. There were placed hidden links on every index page of every website on the server.