My Word Press Blog Does Not Work

I have a word press blog and for some reason when I go to it at: https://www.learnmagictricks.org/blog it no longer shows up. However, there is text there that says an error has taken place. Below is a copy of the text I get on the screen:

Warning: Cannot modify header information – headers already sent by (output started at /home/learnmag/public_html/blog/wp-content/themes/default/functions.php:418) in /home/learnmag/public_html/blog/wp-includes/pluggable.php on line 390

Can someone please help me fix this issue?

Thanks.

David