help!

after i did the stupidity to add a code in fuctions.php to activate my menus(but now erased), when i am trying to activate a plugin, appears “
Warning: Cannot modify header information – headers already sent by (output started at /home4/absolvh8/public_html/wp-content/themes/blissful_blog/functions.php:80) in /home4/absolvh8/public_html/wp-includes/pluggable.php on line 897″
how can i get rid of it?