locked out of site

Tried to update an post and got the following message.

Warning: Cannot modify header information – headers already sent by (output started at /home/content/94/5588594/html/blog/wp-content/plugins/wp-attachments/wp-attachments.php:37) in /home/content/94/5588594/html/blog/wp-includes/pluggable.php on line 934

I was unable to update. Now the same message appears when I try to log in so I cannot do that also.

How do I fix that problem.