The network server (running the website) believes that the HTTP data stream sent by the client (eg your browser) was correct, but access to the resource identified by the URL is prohibited for some reason. This is equivalent to the 'return to sender - unknown address' response to conventional mail services.
Repairing 404 Errors - General
For top-level URLs (such as www.isp.com), the first possibility is that the request for the site URL was directed to a web browser that believes that it never had any page for the website of the network. This is possible if the DNS entries are fundamentally corrupted or if the network server has internal corrupted records. The second possibility is that the network server hosted the network site, but now it no longer does so and can not or will not provide a redirect to another computer hosting the site. If the site is completely disabled - now effectively nowhere on the Internet - then the 404 message makes sense. However, if the site was recently downloaded, then the 404 message can also be triggered. This is also a DNS problem because the old network server can no longer be accessed - as soon as the global DNS entries are updated, only the new network server should be accessed.
For low-level URLs (such as www.isp.com/products/list.html), this error may indicate a broken link. You can easily see this when trying the URL in a web browser. Most browsers issue a '404 - Not Found' message very clearly.
As long as the network site is still found somewhere on the Internet, 404 errors should be rare. For top-level URLs, they usually occur only when there is some change in how the site is hosted and accessed, and even those URLs disappear within a week or two after the Internet captures the changes that were made. For low-level URLs, the solution almost always is to fix the web pages so that the broken hypertext link is corrected.