We see this question often, and 99% of the time it is related to the site owner triggering our anti hack security measures, and locking their IP address from our servers.
One of the way you could test to see if the problem is local or server related, is to use a proxy. A nice proxy to verify if the site is working is at http://netrenderer.com/. This free service allows you to pull up your site in IE7, and other browsers to see what it would look like. While this is designed to be used as a testing site for IE rendering, it works great as a verification engine also.
If you site is coming up in that site, but still not at your location, then you either have a local computer or ISP problem, or you have been blocked.

Keep in mind some of these cases will have IP unblocked automatically after 1h, but depending on the situation, the IP will get blocked permanently. If that is the case, you can supply us with your IP from http://whatsmyip.org and we can check the server and see what is happening.