You can try these two troubleshooting routines but it's a long shot:
1 - delete the iWeb preference file, com.apple.iWeb.plist, that resides in your
User/Home/Library/ Preferences folder.
2 - delete iWeb's cache file, Cache.db, that is located in your
User/Home/Library/Caches/com.apple.iWeb folder (Snow Leopard and Earlier).
3 - launch iWeb and try again.
If that doesn't help continue with:
- move the domain file from your Home/Library/Application Support/iWeb folder to the Desktop.
- launch iWeb, create a new test site, save the new domain file and close iWeb.
- go to the your Home/Library/Application Support/iWeb folder and delete the new domain file.
- move your original domain file from the Desktop to the iWeb folder.
- launch iWeb and try again.