You need to find the most up-to-date version in iWeb itself and you can do this by finding your domain.sites file. This is found in your User/Library/Application Support/iWeb/domain.sites. Ensure that you look in your User Library and not your System Library as you won't find your domain file there.
If you don't have a backup of this file, then there is nothing that you can do because iWeb does not have an import facility, so is unable to import a site that has been published, so no html and css files. You will just have to update the domain file that you can find for iWeb by copying and pasting from your published site.
If you have lost the relevant domain file, then try doing a spotlight search or downloading the Find Any File app and you might get lucky. Other than this there is nothing you can do.
In Lion the Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
To open your domain file in Lion or to switch between multiple domain files Cyclosaurus has provided us with the following script that you can make into an Applescript application with Script Editor. Open Script Editor, copy and paste the script below into Script Editor's window and save as an application.
do shell script "/usr/bin/defaults write com.apple.iWeb iWebDefaultsDocumentPath -boolean no"
tell application "iWeb" to activate
You can download an already compiled version with this link: iWeb Switch Domain.
Just launch the application, find and select the domain file you want to open and it will open with iWeb. It modifies the iWeb preference file each time it's launched so one can switch between domain files.
WARNING: iWeb Switch Domain will overwrite an existing Domain.sites2 file if you select to create a new domain in the same folder. So rename your domain files once they've been created to something other than the default name.