Google periodically issues different algo updates that influence different aspects of website rankings. You can overview the summary of all the major updates here:
http://moz.com/google-algorithm-change
Depending on the update specifics you need to act accordingly: review your link building/outreach methods, optimize your site structure, content, etc. There are lots of guides available for each Google update on the Web.
Also, you can read more news and advice on all the major updates here:
http://searchenginewatch.com/topic/google_algorithm
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