windows server 2003 - Cannot access public facing website

Windows Server 2003, IIS 6

We have a public facing website deployed and until this morning it was running fine.
Now when we try and access the website from outside the network the browser is returning a 'website not found' message. We are not receiving an IIS error message, it's just stating that the website cannot be found.

Accessing the url from the internal network works fine, as does pinging the url and also the ip address internally, so it doesn't appear to be an IIS issue. tracert is just hitting internal servers as expected, so this may be a DNS issue?

Any ideas where to start looking to solve this problem would be greatly appreciated.

Thanks,
Ciaran.

Edit:
We also have another public facing website (with a different IP address) on that same server and that site is accessible externally, so the problem might be related to the IP address of our site?