cannot login error

I was trying to edit the header1 size in wordpress, (simple catch theme) and made changes in the code, and now our website doesn't show up (only some pages), also I am unable to login to the link below, it goes to a blank white screen. http://www.ieefa.org/wp-login.php I was told to use an FTP, however I cannot log into that with the username and password the host provided. I was told we have the following errors. Any advice would be greatly appreciated. -Thanks, C [17-Jul-2013 16:38:33 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /mnt/stor7-wc2-dfw1/538952/762782/www.ieefa.org/web/content/wp-content/plugins/display-text/display-text.php:1) in /mnt/stor7-wc2-dfw1/538952/762782/www.ieefa.org/web/content/wp-includes/pluggable.php on line 876 [17-Jul-2013 16:38:46 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /mnt/stor7-wc2-dfw1/538952/762782/www.ieefa.org/web/content/wp-content/plugins/display-text/display-text.php:1) in /mnt/stor7-wc2-dfw1/538952/762782/www.ieefa.org/web/content/wp-content/plugins/quick-pagepost-redirect-plugin/page_post_redirect_plugin.php on line 1208 [17-Jul-2013 16:41:56 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /mnt/stor7-wc2-dfw1/538952/762782/www.ieefa.org/web/content/wp-content/plugins/display-text/display-text.php:1) in /mnt/stor7-wc2-dfw1/538952/762782/www.ieefa.org/web/content/wp-includes/feed-rss2.php on line 8 I would recommend replacing the WordPress core files via FTP