Please help, can’t access dashboard

Hi There, I was trying to set up a copy of my blog on a local server in order to make some changes without changing anything on the main site. I backed up the database and all the wordpress files before doing anything and then reverted to the back up. But now although my site is up and running I can’t access the dashboard and I get the following error:

Warning: Cannot modify header information – headers already sent by (output started at /home/user/public_html/portfolio/wp-config.php:153) in /home/user/public_html/portfolio/wp-includes/pluggable.php on line 865