BuddyPress.org

If you have FTP access to your site, rename the buddypress folder like this: wp-content/plugins/xxxx_buddypress

This will deactivate the plugin and let you come back to your site admin. Once you have access to your admin, upload latest BuddyPress 2.6.1.1. Before to activate it, go back to FTP and delete xxxx_buddypress.

Now you can activate BP and normally, anything should work.