The sandbox effect or (site getting banned on google) has been prevalent in google for quite a long time, and many websites are falling victim to it.

Whether deliberately or accidentally, these websites get caught by so called (GOOGLE POLICE CRAWLER) and in a similar fashion your site domain goes to a prison cell. While staying in this prison cell for 2-3 or maybe 4 months, google police crawlers revisit the cell to check the status of the website domain.

If the site webmaster or whosoever handling the site notices that the site is not showing on google or any other search engine, and manages to pin point the defect in the site, your website gets back to life, otherwise your site gets life imprisonment for rest of its life, and no one can bring it back, unless you are a hacker and go inside google database to modify your site details.

Normally people ignore POLICE CRAWLERs in the first place, and later find that it's too late. If you detect any fluctuations in google rankings, it could be possible that your site is entering Googles Sandbox.

Google has a rule book of a fickle minded child which changes every 2-3 months, and several sites which do not keep their sites updated with the changes in the algorithm google is using to crawl your site, gets off the track, and ultimately die a slow death.

Its more like a slow poison, where your site dies not in a single day, but slowly, without your knowledge, because google would never disclose openly that your site has indeed been penalized.

Earlier the sites used to get penalized, and were immediately removed from the google search engine. Like for example, I was given task to restore the site www.yxz.com, about 4 months ago

Now some of the problems facing with the site were

1)Their url was not even showing on google.

Now closely examining site page by page I came to know that one of their pages was pointing to their partner site www.abc.com, which received life imprisonment on google.

2) Moreover both were sharing same IP address, and the site received penalty on google.

Now if you are hiding a criminal in your house, you are also a part of active crime, and police will certainly punish you in the same manner, maybe in a mild fashion.

I explained my theory, and recommended them to remove the link pointing to abc.com, and certain redirections on certain page, but they the owner was bit skeptical about what I was talking and gave me a lecture on whats the importance of www.abc.com for their business, and other Business Jargon

"OK I said", why not go for flash link on that particular page. and, problem got resolved and site got restored within a month.

The site was banned for more than a years, and nobody in the company looked into the matter why their site is not showing on google.

Thankfully the site got restored, but unluckily manager decided to close the company site since it has suffered great loss over a period of last one year due to several factors, one of them was their ranking on google, which was bringing them customers 30-50 every month from google and other search engines.

Similarly its important for any WEBMASTER, or the owner of the site to keep track of its rankings on google, because if some updates are done to the algorithm, it could be possible that your site is not adhering to the latest algorithm.

Good Luck