The recent Google Panda ranking algorithm introduced new changes for bloggers.
On April 11th, Google globally released their new Panda algorithm. According to Google, the new algorithm is designed to increase the quality of searches and reduce rankings for low-quality sites. When Panda went global, many websites experienced changes in traffic. Some websites gained more traffic while others lost some. According to various reports in some cases websites lost more than 90% of traffic.
If you were affected by Google's recent update, Google provides insight on how to get your websites back on track.
Google Forum for those affected by the new Panda: http://www.google.com/support/forum/p/Webmasters/thread?tid=76830633df82fd8e&hl=en
Other useful links:
Official Google Blog: http://googleblog.blogspot.com/2011/02/finding-more-high-quality-sites-in.html