I hope someone can help me with this as I have absolutely

I hope someone can help me with this as I have absolutely no idea where to start.

For some (unknown) reason, when I try to login to the WP dashboard, I get this message:

Fatal error: Call to undefined function wp_get_current_user() in /home/networth/public_html/cwn/beta/wp-includes/capabilities.php on line 1281

http://networthadvertising.com/cwn/beta/wp-admin/

and I can't go any further.

Please help!

Thanks,
Chris