Your hosting account requires a dedicated IP address to be able to install an SSL certificate. If you don't have one then your account must be upgraded. As part of the upgrade process your hosting account's DNS settings will be adjusted to accommodate the new IP address. During this time you may experience your website become unavailable, please allow a few hours for the DNS settings to be fully updated.
If after a few hours the site is still unavailable you could try clearing your browsers cache and/or clearing your local DNS cache.
Please get in touch with our support team if you require further assistance.