Recently I visited Google Webmaster Forums (the category Crawling, indexing & ranking ) and spent some time reading all the various posts of people who lost rank and traffic to their blogs, with many cases of sites that have been completely banned by Google, for various reasons.
With all the recent updates of the algorithm ( the last one is called Penguin), there was a real massacre in the web.
Google has started with Panda Update a fight, trying to remove the webspam from their results (SERPs), increasing the experience of the users, willing to reward the webmasters that use White Hat SEO, which try to write high quality content, satisfying the search queries of Google users.
If you have been banned from Google, you may request to be included again, but not before you make all the changes to your site, respecting the rules (checking the Google Official Blog may help you).
So both if your site has disappeared from Google or has lost several positions in Google results, you can check in the list below if you find something that will help you.

Reasons Your Website Has Lost Ranking in SERPs

1 . The content of your website is updated rarely : publish high quality and original content with a scheduled time.
2 . The site does not receive incoming link : check how you can build good backlinks.
3 . The site receives incoming links, but from poor quality websites : use the free tool “Open Site Explorer” , check and manage the links to your blog.
4 . The site has participated in the buying and selling links method : contact the webmaster of the websites where your link is listed and ask to remove it. The problem is also defined as “unnatural linking”.
5 .Your site has external links that point to web sites of poor quality : pay attention to what you link at in your content and if you find it useful, use the “NoFollow” Tag.
6 . The website contains too much advertising, mainly concentrated in the upper part of your pages, at the expense of content : do NOT put ads above the fold. If your site is penalized, you will lose rankings and traffic, so if you do not have visitors, no chance of clicks or views on your ads. Probably it is better to enhance the content, not the ads.
7 . The website contains duplicate content : perhaps one or more sites are also stealing your content. And we all know, Google still has trouble figuring out who is the thief and who is the victim. Use Copyscape and then report the copier to Google. See also other tools to check if your content is copied.
8 . Google is doing a test or changed its algorithm : check what is the update related and make in your site the changes needed.
9 . Google Dance : it is the process by which Google monthly updates its archive pages, recalculating the number of links that each page receives from other (backlinks) and the value of PageRank of each of them. The Google Dance takes a few days. For a long period has always occurred at the end of each month, but its activation does not follow precise rhythms and therefore may be subject to advance or delay.
10 . You website is too slow to load: be sure to install a Cache Plugin, if you use WordPress, so it can cache pages and deliver them without accessing the database, making your site much faster. And if you host your site, avoid hosting services that cause any delays. I use HostGator .

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http://www.freemakemoneyadvice.com/2012/04/30/18-reasons-why-your-blog-is-losing-rank-in-google/