The best thing to do is just re-install the theme.
You can do this by going to “Appearance > Editor”, and changing the version number in the header of the style.css file to something less than what it is now.
WordPress.org will then think you’re using an old version, and will prompt you to update.
You should also check all of your other files, and change all of your passwords (WP, Hosting, cPanel, FTP etc..).