If you're trying to access and edit one of your websites, but the site won't load, it's most likely a problem with your domain name. Chances are your domain was not connected properly and you'll need to review the instructions on How to Connect a Domain to see where you went wrong.
In the meantime, you can always access your site by clicking on the "Demo URL" link from your Dashboard. This URL should always work, even if the site's custom domain does not.