Another great example of Google randomly removing sites based on flimsy or nor evidence. The fact they don't give you a chance to plead their case is pretty scary.
My hunch is one of your competitors got the site taken down. I've had a few clients who had their competitors file complaints with Google, or inform them they were using black hat SEO to get on the first page of the SERPS. In one case they were successful, but I had the site back up in less than 24 hour,s so it wasn't a huge deal.
It sounds like it might be an ongoing issue and are not going to put your site back in the SERPS for a while. I feel for you brother.
But often there is a reason why a site vanishes from the Google index. We would have to exclude all possible violations of the Google guidelines, before we could safely claim that this is a great example for random removal by Google.
Edit: In a comment above Matt Cutts from Google gives an explanation.
My hunch is one of your competitors got the site taken down. I've had a few clients who had their competitors file complaints with Google, or inform them they were using black hat SEO to get on the first page of the SERPS. In one case they were successful, but I had the site back up in less than 24 hour,s so it wasn't a huge deal.
It sounds like it might be an ongoing issue and are not going to put your site back in the SERPS for a while. I feel for you brother.