Help needed please. Cannot modify header information - headers already sent by

[Phil Dart]

Hoping someone can help

Site suddenly went down in the small hours of the night. No files had been changed.

The error message is

Cannot modify header information - headers already sent by (output started at /home/s*****/public_html/dbtech/dbseo/includes/class_db:865)

I've spent plenty of time on google, reading about white space, which I cannot find, and in any case, the site was fine one moment, than failed the next.

Version is 4.2.3patch

Line 865 in public_html/dbtech/dbseo/includes/class_db is
if (!$connect = $this->functions['connect']($link, $servername, $username, $password, '', $port))