Cannot access Page caused by Fatal Error

Hi all,
I have been completely disabled from editting my site somehow.
I have had a developed site for over 2 years and went to try some new themes. All of a sudden i got this error message:

Fatal error: Call to undefined function wp_enqueue_media() in /home4/radmanx/public_html/prismaticmantis.com/wp-content/themes/virtue/lib/cmb/init.php on line 257

Now I can only see the undeveloped version of my site with this new theme. And where ever I click to sign in through the back end or edit anything, it takes me to the same blank error page.

http://www.prismaticmantis.com

What has gone wrong? How can I fix this?

Please help !! :/