Wikipedia pages can be edited by anyone; they can also be deleted entirely by Wikipedia administrators for a variety of reasons. Within the Wikipedia community there are differing views regarding the retention and deletion of articles, e.g., see the Wikipedia article Deletionism and inclusionism in Wikipedia. If you read an article and want to ensure you have access to the information in the future, the best course is probably to use Wikipedia's own "print/export" feature or to save the content of the article elsewhere by using a service such as Evernote, which provides notetaking and webpage archiving services, Diigo, which provides a social bookmarking service with the capability to store a copy of webpages you have visited, i.e., to "cache" them so that you can view the webpage again as it was when it was cached whether it has been changed or deleted in the interim, or similar services. But, if you haven't archived an article and find a Wikipedia page was deleted and so is inaccessible to you, there are still some options available to you.
If the article was deleted some time between February and September 2008, you may be able to find it on Deletionpedia at deletionpedia.dbatley.com Deletionpedia is an archive that contains 62,224 pages which were deleted from the English-language Wikipedia between February and September 2008. If you know the title of the article that was deleted, you can browse "Pages deleted after more than 1000 days on Wikipedia" or " Pages edited more than 200 times" by their alphabetical listings. If you know the date the article was deleted which, if you have the Wikipedia URL for the article, can be found by visiting the URL for the article on Wikipedia, you can find it by searching Deletionpedia by Pages by deletion date.
Deletionpdedia's own search feature is disabled and the site suggests you use Google to search Deletionpedia. However, I've found that approach is likely to miss articles stored on Deletionpedia. E.g., Deletionpedia contains the article Elvis sightings (deleted 03 Jul 2008 at 10:12), yet if you search the site using Google with Elvis sightings site:deletionpedia.dbatley.com, no results are returned. Incidentally, Wikipedia does now contain an Elvis sightings article.
Interestingly, though Wikipedia now contains a Deletionpedia article, that article was itself once deleted from Wikipedia.
Another site Fixed Reference: Snapshots of Wikipedia provides access to articles archived in April and July of 2004.
Because Fixed Reference and Deletionpedia only provide access to articles from two years, 2004 for Fixed Reference and 2008 for Deletionpedia, their usefulness for accessing deleted articles is very limited. Another alternative is to search the Internet Archive at archive.org. The Internet Archive is a non-profit digital library with the stated mission of "universal access to all knowledge." It also archives pages found on the World Wide Web (WWW). The archived pages, which are created for a website when the Internet Arhive periodically scans the site, are accessible through its Wayback Machine. The name is a reference to the time machine used by Mr. Peabody, a talking dog, and his human companion, Sherman, in the cartoon series The Rocky and Bullwinkle Show to visit famous events in history. You can choose to "Browse History" to search for an archived copy of the page deleted from Wikipedia, if you know its URL. If the page was archived by the Internet Archive multiple times over a period of time, which could span years, you can view the page as it was on the particular days it was archived.
Another place you can check for deleted Wikipedia pages or pages that have disappeared from any website is Archive.is, which aims to be "your personal Wayback Machine!" The site can be freely used by anyone to take a "snapshot" of a webpage that will always be online even if the original page disappears. So, if someone else has archived a particular webpage for which you are searching, you may find it at Archive.is. You can also use the site to archive pages you may want to access later that could disappear from the original site or to make the pages available should the original site disappear from the web.