What kind of server is your website on? Linux or windows? There was a rather nasty linux worm recently that shut down at least one entire subnet that I know about. Some of the IPs on that subnet were down for almost a week. Not sure if it has anything to do with your problem though.
As ISD and SE has said a link and a backup of the site might be helpful. In fact any information you can give on the problem will be helpful. It's really kind of hard to know how to advise you with the little information you have given.