First of all you need find out the cause of the hacking ( in most of the cases website hack due to insecure plugins and theme, badly written scripts, and weak password) and then take steps to fix the issues. Once you have done that, your website will backup and running as it before.
I have Joomla content management system. My joomla is hacked by some one. What can I do now?
This happens mostly when you do not have proper tweaks done on the server. Make sure you do not have incorrect permissions set on the files and also you are running the latest version of Joomla.