Warning: Cannot modify header information

Suddenly this warning pops up:

Warning: Cannot modify header information – headers already sent by (output started at /home/www/fancyskirts.dk/wp-content/plugins/easy-facebook-likebox/includes/easy-custom-facebook-feed-widget.php:3) in /home/www/fancyskirts.dk/wp-content/plugins/wpfront-notification-bar/classes/class-wpfront-notification-bar.php on line 68

What to do?