Unfortunately there is no quick and easy method to reconstitute an iWeb domain file from raw html. For future reference, you may either want to backup automatically with Time Machine, SuperDuper!, or Carbon Copy Cloner, or do a manual backup of the iWeb project file by digging into ~/Library/Application Support/iWeb/Domain.sites and pulling that to a flash drive or external drive. Double-clicking on the relocated Domain file from the alternate drive will set that as the new active file in iWeb and will allow you to even work on your site from another Mac as long as it has the same or newer version of iWeb.