If you already have the certificate on the Default Web Site, it should work unless there is an issue with certificate.
Check if you have 'Private Key' assigned to the certificate. You can click on 'View Certificate' and you should be able to see it under the 'Valid From'. You will be able to see this only on the Default Web Site.
Also try using
HTTPS://IP_address_of_server and check if that helps...
let us know how it goes...