Changing domains

Hello,
I’m in the process of converting a static html site into WordPress; Because my html site is live/active, I started building the WP site on an extension of the domain (for example: mysite.com/new). When I completed all the pages etc., I went into the settings menu and changed the url to mysite.com. I got a 404 message, and I can no longer access the wp-admin portion of the new or old site designation. It appears to be gone. My questions:

1. How can I fix this?
2. How can I accomplish this correctly (build site in WP and have it replace the <mysite.com domain/current html site)?

Thanks in advance.