When you access your custom domain you don't see your Heroku application.
This error can occur for a number of reasons.
Custom domain not added to application
You may not have added the custom domain to your Heroku application. Ensure you have added each variation of your domain that you wish customers to access your site via, eg. mydomain.com and www.mydomain.com - read more.
Once you've added the custom domain to your application you need to configure your DNS to point at your application's DNS Target - read more. We also have a complete guide on adding domains and a guide specific to root domains and common caveats with these types of domains.