The Web links documents to one another, but -- by accident or design -- some pages are occasionally lost in the shuffle. Sometimes a Web page edit deletes the last link to another page and now it's hidden. There's a good chance you can still find it unless it has been removed from the Web server.
Use Google's Site Search
Use Google to search for the page but add a restriction on which sites the search engine will return. For example, search for "iPhone site:apple.com," and only pages on the Apple website come back in a search. This is the easiest way to find a page that has been accidentally hidden; use Google to search a specific site for text that you expect to appear on the missing page.
Try a Directory Listing
Some Web servers return a list of all files in a directory if you ask for a URL ending with a "/" character. If you think you know which folder contains a missing file, you can try that as a URL. For example, if you're looking for "http://www.somesite.com/members/members.html", try "http://www.somesite.com/members/" and see if a directory listing comes back.
Modify the URL
Some URLs make it obvious which parts can be modified to look for other files. If you're on the page "http://www.somesite.com/2014/index.html", try changing the 2014 to another year to see if a prior year's pages are still on the site. This might also work with other kinds of serial numbers in a URL.
Contact the Webmaster
Depending on what's gone missing and whether you think the publisher meant to remove it or it was lost by accident, the webmaster of a website is the person who knows the most about what's on the site. Most websites publish an email address or form for the webmaster; try contacting him and asking about the page you're looking for. If the webmaster meant it to be published, the page should be back on the Web with an obvious link to it pretty soon thereafter.
Explore the Deep Web
The Deep Web refers to websites that do not show up in standard search engines and that escape the notice of the normal techniques used to categorize and index the Web. Some of these sites may deliberately avoid indexing to provide illegal or gray-market services, but most of the Deep Web consists of databases of information that doesn't exist in Web page form until a particular search produces a page of results. Deep Web resources can sometimes be found by researching the surface Web that appears in search engines or by using search engines designed specifically to search the Deep Web.