On Sunday, if you searched in Google for [Green Party] you would not find the official Green Party of United States web site, which is at gp.com. Even if you searched for gp.org, Google would not show you the site.
One person complained about the issue at Google Web Search Help forums and a few days later, a Googler came in and told us why.
In short, the site was hacked and included harmful malware that could infect searchers computers. Google removes sites that include malware from their index, until the site is fixed. In addition, if sites are injected with links to unrelated site, with the sole intent of link spamming Google, Google will also remove the site from their index. That is what Google did in this case, and now the site is back in the index.
Googler, Jaime said:
The gp.org website was removed from Google's results because it has been hacked (if you look at the source code for http://gp.org/, about halfway down you'll see hundreds of spammy hidden links to websites selling several drugs such as Viagra and Fluoxetine).
We sent an email to the gp.org webmaster a week ago, on Monday the 19th, and they were also notified via the webmaster tools console (http://google.com/webmasters). Anyone in contact with the owners of this site, please give them this information and, as danielroofer pointed out, let them know they can visit the Webmaster Help Forum if they have additional questions.
In the meantime, we've already reinstated this site into our index, but it may take up to 24 hours for it to start showing everywhere.
Today, it seems like the spam and hack has been removed from GP.org and the site is now back in the Google index.
Forum discussion at Google Web Search Help.