It is usually because there are spaces, new lines, or other stuff before an opening tag, typically in wp-config.php. This could be true about some other file too, so please check the error message, as it lists the specific file name where the error occurred (see the link below for more info on “Interpreting the Error Message”). Replacing the faulty file with one from your most recent backup or one from a fresh WordPress download is your best bet, but if neither of those are an option, please this codex: http://codex.wordpress.org/FAQ_Troubleshooting#How_do_I_solve_the_Headers_already_sent_warning_problem.3F
