Help! WordPress update incompatible with theme?

On my posts page I get this fatal error: Fatal error: Call to undefined function get_the_post_format_url() in [[my directory….]] killingwilliamsburg.com/wp-content/themes/vertigo/content-link.php on line 6

Hostgator says that the new wordpress is incompatible with the theme, but I’ve been unable to contact the theme developer. Hostgator doesn’t offer a back up old enough to predate the update. and i don’t want to kill the theme.

Any way to rewrite the code to fix? Or any suggestions as to how to DOWNgrade my wordpress? which versions will work?

This all worked like a dream for the last year. thanks,