Developers

Check your DNS settings

It's possible that your root domain (mysite.com) could be pointing to a different website than the WWW version, (www.mysite.com). To correct this, pull up your site's DNS records. Verify that both the root domain points to the same host as your WWW domain. The WWW domain record should either point to the same IP address as the root, or, a CNAME could be used to point to the root record.

Flush the DNS cache

If when visiting your site's domain in your browser, you are still seeing your old website come up (even hours after the correct DNS settings were entered), it may be that your computer or network is still using the old (cached) DNS information for your site.

DNS records have a "Time To Live" (TTL) setting which tells networks how often they should be refreshed, however it is possible that the old information could still remain, depending on your network configuration. 

In this case you can flush (reset) the DNS cache to make sure your computer is requesting the latest DNS changes.

Windows
How Do I flush My Windows DNS Cache?

Mac OS
Reset the DNS cache in OS X

Linux
How To Flush Linux / UNIX DNS Cache 

To flush the DNS cache for your entire network, contact an IT professional.