It sounds to me that you have somehow lost the original file you created. You need to figure out if the file that iWeb uses is still existing.
Open a Finder window to your home folder.
Open the Library folder within that.
Open the Application Support folder.
Open the iWeb folder within there.
So; username->Library->Application Support->iWeb.
In that folder should be a file named Domain or perhaps Domain.sites2 as it is for me. If you do not see such a file there, then use Spotlight to search for the word Domain and if it exists, it will be listed under the Documents group.
An alternative in Spotlight is to search for the string DomainState.archive which will be contained in the Domain file. You could then click on Show All in Spotlight and a list will come up and you could click on the little round i icon to the right of the file name to find the path of your Domain file.
If you find the file, this
Apple link (assuming iWeb 1.1 or newer) says you can double click a Domain file to make it the default file opened when opening iWeb. I should think that opening via the Spotlight listing would do the same thing.
If your Mac is set to another language, perhaps the names above will need some translation from you to understand. I'm not sure as I've only used English.
I've made some assumptions; First you haven't done a complete reinstall of OS X since making that web page, second you are using the same account name and computer as when you made the original file, third you didn't delete things on purpose or by accident.
I have on occasion seen people complain that they have lost there iWeb work by unknown means. I'm not sure they ever got it back, so you might not. This is why backing up are important. If you are using OS X 10.5 (Leopard) than I suggest you buy an external drive for use with Time Machine.