We are used to the web working without any problems but there are actually lots of different 'points of failure' any of which can block a web site.

The problem of course is knowing what's causing the problem. http://downforeveryoneorjustme.com is a very simple web site that checks if a web site is working or not from another computer. Enter the problematical web site in the box and click on the or just me? link. I like simplicity like this.

And if can't access this web site, it's a fairly good indicator that the problem rests with your computer...