Website Is Down - Receiving 404 Error - Help ASAP

Hi there, our website is down and I'm receiving a 404 error.
I receive the same error when I try to log into the admin, can't get anywhere.
http://www.htkmarketing.com
http://www.htkmarketing.com/wp-admin.php
What can I do? and how can I prevent this in the future?
Thank You,
- Mark