You will need to re-configure the website to work over HTTPS. You can try forcing a redirection to HTTPS from the Control Panel -> SSL/HTTPS -> Force HTTPS -> click on the "Enable" button next to the hostname in question. In most cases the redirection to https:// will work without issues.
However, if you have included in your site images or other resources that are loaded via hard-coded http:// links, you will need to manually edit these links, so that they load via https://. Otherwise, you will receive mixed content warnings in some browsers.
If you are using WordPress, the "Force SSL" option in the Control Panel might not be sufficient. In such cases, you can refer to our blog post for instructions on how to move WordPress to HTTPS, or you can contact our Support Team.