If the following image displays when you visit your website, it's been blacklisted by Google Safe Browsing:
To figure out why this happened, you should visit Google's diagnostic page for your domain at http://www.google.com/safebrowsing/diagnostic?site=[your domain name]
On this page, you can see the current status of your domain, the date Google scanned it last, details regarding its suspicious content, and instructions for what to do next. You can use this information to assist you in determining how to remove the offending content from your site.
It's important to note that the information you see on the diagnostic page might be out of date because Google does not visit every site on the Internet every day.
If you've resolved the issues Google found, you'll need to request another review using Google Webmaster Tools®, which you can access here.
Google also has a walkthrough containing all of the information you need to resolve a compromised site at google.com/webmasters/hacked/.