Google gives users a warning on the search results if a website had badware. The easiest way to find out if you are listed is searching for your website in google and see if a message appears: "This site may harm your computer". If so, you can sign up at google webmaster tools (http://www.google.com/webmasters/) and request a review and possible see 'infected' URL's. Google traffic drops about 70% when users see that message. Experienced it myself when users on my forums were linking to 'bad' imagehosts. Solved it within a week. I've marked the area when my site was listed as 'infected'.
Please check all of your backlinks. you can also check that is there any problem occured in your hosting server like is your server temporarily unavailable.