When a site is transferred from one IP to a new IP people who have recently visited the site will likely have the old DNS information cached and will be unable to see the new site for a period of 4-48 hours.

Sites are transferred to new IP's either when moving from one server to another, or when installing an SSL certificate, or when experiencing email delivery issues and we have determined that you could receive better email delivery rates with another IP. Remember, DNS caching only affects someone who has recently visited your site; and any new visitors will not experience this issue (so your active members will typically have the issue; their contacts will not have the issue).

The links below explain what can be done so you do not have to wait several hours for your DNS cache to clear on it's own.

Below are 2 pages explaining how to clear the DNS cache:

Clearing Browser Cache - Wiki

Clear Windows 7 Cache