If you get lines like this on your website pages instead of the website then someone edited the first file referred to in this error and they added extra lines and spaces behind the last closing bracket of the PHP page.
Warning: Cannot modify header information - headers already sent by (output started at /home/colin/domains/yoursite.com/public_html/wp-includes/default-filters.php:214) in /home/colin/domains/yoursite/public_html/wp-login.php on line 290
The "headers already sent" error is usually caused by having white space before or after the opening and closing PHP tags (<?php . . . ?>).
Have you recently edited any files?
Remove any extra spaces and lines after that last bracket on that page, save the file and try again. It should be fixed.