You can try immediately and repeatedly, or you can wait twenty minutes, or you can wait until a slower time of the day. It may mean that the page you are trying to access is more popular than the Web site can handle, or it may mean the Web site is too large for the computer it is located on to hold. Most of the time these messages are temporary, and they may appear even when you try to reach the same document you just visited. That you saw it recently is no guarantee that it exists any more or that it hasn't moved without a forwarding address, but usually the message indicates the site is temporarily down or the network connecting the two sites is experiencing technical problems. However, if you have never visited the web page before, there is strong possibility that the link no longer exists.
Web pages come and go all the time.
Links need to be updated frequently, but few web page creators have the time to test their links on a regular basis. Good Web page creators should tell you when the page was last updated. If a link continues not to be working, check the date of the page on which the link is found. But don't give up too soon on links. Try them a few times to be sure you can't get through.