Sometimes sites are not accessible maybe because they are moving to a new webserver, have problems with their hardware or simply get slashdotted / digged and can´t handle that many requests which leads to timeouts for the user. This happens of course most of the time when you desperately need the information on that website.
There could be a way to see the contents of the website even though it is not reachable at the moment or dead forever. Some services, mainly search engines, store cached versions of a webpage. You can check a cached version by searching for the website on google, yahoo or msn and select the cached version of the website instead of clicking the link to the homepage.
Coral Cdn is another service that lets you view cached pages by simply appending nyud.net:8080 to the request. All of this can now be accomplished with the firefox extension resurrect pages. The extension offers five different cache providers: Google, Yahoo, MSN, The Internet Archive and Coral CDN.

View Content of The Page cannot be displayed websites
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There is also a Firefox extension called Cache It! that’ll goto the google cache just by right clicking on the page or clicking on a toolbar button.