Imagine you wake up one morning and find your site that pays your mortgage, supports your family and the families of your employees have been hijacked by Chinese hackers. That would be scary.
That happened to ShadesDaddy.com, a site that sells sunglasses, on February 3rd. His whole business was taken from him he said in a Google Webmaster Help thread:
I am seeking immediate help / advice. My website, www.shadesdaddy.com, which I've owned for 8 years (has taken me 8 years to build, ) millions in revenue, employees (feeds a few families,) has been hacked by chinese hackers. The domain was officially stolen (removed) from our registry account on FEB. 3, 2015.
Google's John Mueller responded a bit late but he said, "there's really nothing Google can do when a domain changes hands like this, it really needs to be handled at the source."
He was able to get his site back, via the source, and now that he has it back, John has given him some advice at getting his rankings back, so he can get the same traffic from Google he originally had.
John said:
(1) You want to make sure the content that Google has not brought back into the index is brought back. To speed it up, John said use "sitemap file or the "Fetch as Google" & "Submit to index" feature in Webmaster Tools for the URLs that are still missing."
(2) If you want to dig deeper, he said, then analyze the log files. He said:
You could try to analyze your old log files (which unique URLs were accessed by users with a Google search referrer) and compare that to the new log files (which unique URLs were accessed by users with a Google search referrer). The difference ("before" minus "now") could be used to compile a list of the missing URLs, which you could then either manually fetch & submit, or submit as a HTML sitemap page.
I hope this never happens to any of you - it is so sad to read stories like this. I am just glad he got his site back and is working on returning his traffic and revenues.
Forum discussion at Google Webmaster Help.