The search industry is one in which the rules are constantly changing. The major search engines (Google, Yahoo! and Bing) are constantly adjusting their algorithms to remove and prevent what they consider spam (low quality) sites from appearing in their search results.
Unfortunately these changes often have a negative impact on quality sites. Recently Google released their newest update called 'farmer' or 'Panda'. This update, while achieving their goal of removing a number of low quality sites from the Google index, also removed a number of quality of sites from the index.
Unlike previous updates by Google it seems like this one is not going to be reversed or under go a major rework.