Help needed asap. Getting error message

Hi,

I logged in to the dashboard and noticed that there is an update for the automatic upgrade, so I clicked on it and now I can’t login to my site, can’t see my site, can’t do anything.

All I see in the browser is this:

Warning: Cannot modify header information – headers already sent by (output started at /…/…/…/blog/wp-content/plugins/wp-phpbb.php:69) in /…/…/…/blog/wp-includes/pluggable.php on line 770

What can I do to fix this?

Please, any help asap is much appreciated.

Thanks!