I would guess that you hosted your site on MobileMe? - the URL was web.me.com/something.
As was announced over a year ago and in a number of emails since then, MobileMe closed on 1 July and your site is no longer available there.
You will need to find another hosting service - there are plenty to choose from. Up to and including iWeb '08, it was only possible to publish to other hosts by 'Publishing to a folder' and then uploading that local folder to a website host using a separate FTP client such as Cyberduck or Transmit. From iWeb '09 an FTP method was built-in, making it easier to publish to a host other than MobileMe, and all you need to do is choose FTP from the Publish pane.
More details here:
MobileMe has been discontinued by Apple & iCloud does NOT allow you to host your web sites. You can continue to use iWeb but, will need a 3rd party web host to upload your websites to. Depending on the version of iWeb you are using you have a couple of publishing options:
iWeb '09 (3.0.4) you can publish to an FTP Server or a local folder. ( With the built in FTP in iWeb you will end up with an address like "www.YourDomain.com/sitename/Home.html )
iWeb '08 you can publish your website to a local folder
I have been using IX Webhosting for several years with little to no problems "usually the problem was something I did" and they have been quick to solve any issue I have come across. The have plans that start at $3.95 a mo (USD) and their customer service is top notch.
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