Site hacked

[vision2000]

Hi,

Someone from Algeria replaced my agency site with their own site. I temporarily replaced the core index.php file with an under construction page.

Questions
How can I prevent this from occurring in the future?
What steps should I take to secure the site?

Thanks

Note from my web host:

We did a scan on the site and we were able to find that the following file was infected:

public_html/wp-content/themes/genesis/404.php

This seems to be an issue with wordpress account hack. We want to let you know that the server is secure and we are using the most latest and updated malware detection tools. Please use only trusted and latest themes and plugins for your wordpress accounts.

We could see that the file 404.php was uploaded from the ip '2.185.216.96' which we have blocked in the server. Also note that we run regular malware scan cron in the server to detect infected files.

Question
How can I prevent genesis themes from being hacked in the future..I thought genesis themes were secure?