Help - site won't load, getting error: Warning: Cannot modify header information

We have an SSL certificate and are trying to secure all of the content on our site. We used the WordPress https plugin http://wordpress.org/extend/plugins/wordpress-https/ .. when some areas of the site were still not showing as secure, we added this code to the functions.php file:
define('WP_HOME','http://simplymusicteachers.com');
define('WP_SITEURL','http://simplymusicteachers.com');
Now the site is broken and we are unsure of how to fix it. It seems to be looping or trying to redirect somehow.

Anyone have any advice??

Thanks!

SM

site url: http://www.simplymusicteachers.com
theme: Blogs MU
WP Multisite install