Sorry to hear your work is lost. This is not a typical Apple experience overall but unfortunately it is typical in this scenario. When or if you have saved your iWeb outside the default file folder Library/Application Support/iWeb, when Apple updates iWeb it often forgets where you were pointing iWeb to originally.
This is the third time it has happened to me, luckily this last time I didn't count on Apple to live up to their brand promise (they usually do, but of all things not to, people's pictures, memories and websites are something you don't want to mess with as you all know).
I backed up my site using Time Machine, unfortunately the last back up was April 27th so I'm still losing months of blogs about my daughter (which someday I was hoping to share with her when she's older). Disappointing that they can't get this right for a few people.
If you moved your domain.sites or domain.sites2 files to another location aside from the default you can try to find them and double click them to open. If you can't find them, you're SOL without a back up. All in all, my only terrible heartbreaking experience on a Mac.