Can’t access site after plugin activation

You’re getting the error because the Facebook and All In One Event Calendar plugins are both declaring the fb_admin_dialog() function, but PHP only allows a function to be declared once.

To fix the immediate problem, log in to your server with S/FTP and rename the wp-content/plugins/all-in-one-event-calendar directory to something like all-in-one-event-calendar-temp; that will deactivate it and remove the error. You can also disable all plugins with phpMyAdmin or another database management tool, if you prefer that over the S/FTP method.

If you want to use both plugins, I’d recommend creating posts on their wordpress.org support forums, and asking the developers to check if the function has already been declared before trying to declare it. e.g.,

if( !function_exists( 'fb_admin_dialog' ) )
{
    function fb_admin_dialog()
    {
        // ...
    }
}