Unfortunately due to the popularity of WordPress it has become a target of hackers scripts. These are used to gain access to the server and use it for any number of uses, such as sending spam, bitcoin mining and hacking other sites of course. The usual method is exploiting bad passwords and default admin usernames or exploiting known vulnerabilities with plugins or themes.

Your site will have some malicious files added by the hacker. What we will need to do is:

1. Rebuild the site with a fresh copy of WordPress and plugins files
2. Manually inspect your theme files to make sure there is no malicious code added to them
3. Host the site on a dedicated server and monitor it closely for any recurring attacks