How to remove the "This site may be hacked" on google search result about your website domain
1. Sign-in to Google webmaster tool https://www.google.com/webmasters/tools/home
2. Register your domain
3. Go to Tab - Securities Issues : see and analyze the detected issues by google
4. search on browser - site: domainname
5. Manually check each file your site if there is an injected code
6. Check each website admin page if there is code that may identified as unwanted code.
7. Run malware scanner like Sucuri and other website malware scanner
8. When code and issues if fix go back to google webmaster tool and request to google to review your site. Please specify how you managed to fix the issue.
This site article may also help you: https://aw-snap.info/articles/may-be-hacked.php