I'm sorry your site is damaged. If you are sure there is malware, carefully follow this guide. When you're done, you may want to implement some (if not all) of the recommended security measures.
If your site contains a theme other than the one you modified, you may want to try this first: Access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue (theme functions can interfere like plugins).