If your domain name expired, then your site will show a advertisement page from the registrar.
Expired domains from GoMedia will look like this:
A whois lookup will reveal that the domain expires in the future, but was updated a day after it renewed, and the nameservers are changed. (The nameservers will automatically revert back when the registration fee is paid.)
Name Server: DNS1.NAME-SERVICES.COM
Name Server: DNS2.NAME-SERVICES.COM
Name Server: DNS3.NAME-SERVICES.COM
Name Server: DNS4.NAME-SERVICES.COM
Name Server: DNS5.NAME-SERVICES.COM
Updated Date: 29-mar-2009
Expiration Date: 28-mar-2010
You have 29 days to pay for your domain name and keep it (at regular price). After 29 days, it will cost extra to renew ($250 plus regular price), and you risk losing the domain to another buyer.