« MODx Parse Error »

MODx encountered the following error while attempting to parse the requested resource:
« PHP Parse Error »
PHP error debug
  Error: Cannot modify header information - headers already sent by (output started at /var/sites/m/mondochallenge.co.uk/public_html/index.php:44) 
  Error type/ Nr.: Warning - 2 
  File: /var/sites/m/mondochallenge.co.uk/public_html/manager/includes/document.parser.class.inc.php 
  Line: 1059 
  Line 1059 source: header('HTTP/1.0 503 Service Unavailable');  
Parser timing
  MySQL: 0.0077 s(0 Requests)
  PHP: 0.0589 s 
  Total: 0.0667 s