Dan Thies has a great blog on how your site can get hacked out of Google. This can literally wipe out your web page from Google’s search results.
Read about it here http://www.seofaststart.com/blog/google-proxy-hacking

While you’re there, you may want to download Dan’s free ebook “SEO Fast Start”. It’s an excellent ebook! You will also receive a six-week video course on building links along with the ebook. Download SEO Fast Start here.