Causes
A link may become broken for several reasons. The simplest and most common reason is that the website concerned doesn't exist anymore. The most common result of a dead link is a 404 error, which indicates that the web server responded, but the specific page could not be found.
Some news sites contribute to the problem of link rot by keeping only recent news articles freely accessible, then removing them or moving them to a paid subscription area. This causes a heavy loss of supporting links in sites discussing newsworthy events and using news sites as references.
Another type of dead link occurs when the server that hosts the target page stops working or relocates to a new domain name. In this case the browser may return a DNS error, or it may display a site unrelated to the content sought. The latter can occur when a domain name is allowed to lapse and is subsequently reregistered by another party. Domain names acquired in this manner are attractive to those who wish to take advantage of the stream of unsuspecting surfers that will inflate hit counters and PageRanking.
A link might also be broken because of some form of blocking such as content filters or firewalls. Dead links commonplace on the Internet can also occur on the authoring side, when website content is assembled, copied, or deployed without properly verifying the targets, or simply not kept up to date.
Read more about this topic: Link Rot