@avinanrey just log onto your server via FTP, and rename the BuddyPress plugin directory, or delete it. This works for any malfunctioning plugin. It will be immediately disabled.
I have come across the same error with WP 3.1 beta 1. Apparently WordPress normally ignores this error.
Fatal error: Call to a member function is_page() on a non-object in /public_html/wp-includes/query.php on line 419
But the issue lies with the plugin.
https://wordpress.org/news/2010/11/wordpress-3-1-beta-1/
So BuddyPress needs to fixed.
