Cannot modify header information - Joomla! Forum - community, help and support

it weird, earlier today moved login block, checked , looked fine. later tonight when went check site had error top. can't remember changing anything.

warning: cannot modify header information - headers sent (output started @ /customers/xxxxxxxxxx.com/xxxxxxxxxx.com/httpd.www/index.php:2) in /customers/xxxxxxxxxx.com/xxxxxxxxxx.com/httpd.www/includes/joomla.php on line 697

i'm looked around answer question, , says spaced before or after ?>
is problem in joomla.php? i've checked templates index.php , main index.php

what else can do?

thanks