If you see the following warning when you visit your site:
This site may harm your computer.
this means that your website has been marked by Google as harmful because it contains malicious code.
The first thing to do is remove all the badware/malicious code from your pages and fix the security vulnerabilities that allowed the code to be inserted in your files. Note that it is important to fix those vulnerabilities to prevent such issues from happening in the future.
You can try cleaning your files by yourself. You should always keep your applications up to date, so after removing the malicious code, an upgrade of your scripts is highly recommended. Alternatively, if you experience difficulties cleaning the files yourself, you can hire a professional for further assistance.
Once the cleaning of your website is completed, you can check your website with the Google Diagnostics page. Just visit the Safe Browsing Site Status page and place your domain name in the Search bar.
After you have done all of the above, request a review from Google’s Webmasters Tools. If you don’t have a Google Webmasters Tools account, you can create one for free here. Once you have created a Google Webmasters Tools account, go to the Overview page and click on the Request a review link.
Then, follow the instructions on the page in order to file a Request for review of your website. Bear in mind that usually it takes approximately 10 business day before your request is reviewed and the status of your website is changed.