iWeb cannot open html files: it can only work from the 'Domain' file, so as you have lost it you will either have to manually edit the html files or reconstruct the site from scratch in another program.
You can open your html files in a number of programs (including TextEdit), but your best bet is probably Kompozer, which is free - it's both a WSWYG and html editor. However iWeb creates very complex and messy html pages, heavily dependent on external css files, and you will find it extremely difficult to make any changes other than simple alterations to text passages.
What happend to the .site file? Did you upgrade to Lion or Mt. Lion recently? If so the following may be of help to you:
In Lion and Mountain Lion the Home()/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 Mountain 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 in your Home/Library/Application Support/iWeb folder that 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.