Guest Post- Many webmaster give the facility of guest posting which are related to the website theme and content of post must be unique.
You should check your associated webmaster Gmail address as your site might get black-listed according to Google policy. Otherwise such sudden change never happen. Over email address, you will get appropriate actions to be taken to recover.
Google panda update 24 has rolled on and it has affected many website. May be this would be the reason why this happen.