Error after upgrading: Warning: Cannot modify header information

I recently updated my wordpress to 3.1 (3 days ago) and now my site is down this is the error I’m getting:

Warning: Cannot modify header information – headers already sent by (output started at /home/shawn02/public_html/wp-content/themes/shawn/custom/custom_functions.php:1) in /home/shawn02/public_html/wp-includes/pluggable.php on line 897

I have read previous threads with similar issues but not no avail nothing is working… can someone please help me??