People are reached our site throgh search engines.Major search used nowdays is google and they have theire own mechanisms for identifying each websites.
For this purpose they developed a tool called webmaster tools.Virus attack of website is a common and regular issue in internet.
Once google identified as your website is malicious and unsafe to visit,they are unindex your site for visitors safety
For again reindexing or reconsidering your website need to follow the steps
1.Login to your webmaster tools
2.Make sure that your website account is existing
3.Request for a reconsideration or reindexing