If you find your WordPress blogs suddenly deindexed by the search engines, you will lose all search engine traffic. However, you can fix the problem easily, though I am still looking for a permanent solution.
In the WordPress Privacy Settings, you have 2 options for
Blog Visibility
I would like my blog to be visible to everyone, including search engines (like Google, Bing, Technorati) and archivers
I would like to block search engines, but allow normal visitors

In certain versions of WordPress, the default option is ” I would like to block search engines, but allow normal visitors” . The blog is deindexed and when you view the source code of the home page , it will have the tag “noindex, nofollow” .

I have manually changed the blog settings to “I would like my blog to be visible to everyone, including search engines”, but even then some blogs are reverting to the deindexed option. Could be a hacker at work or a bug in WordPress / Fantastico.