fatal error in email notification, shut down my site!

this is what comes up when one tries the URL:

Fatal error: Call to undefined function email_notification_future_send() in /homepages/23/d283650616/htdocs/wp-content/themes/elegant-grunge/header.php on line 18

I was trying to ‘fix’ a problem with the email notification subscriptions – I didn’t change anything, but must have messed it up. email notification doesn’t even show up as a tool on my admin menu. thoughts?