Go to (user)/Library/Application Support/iWeb and use Time Machine to restore the 'Domain' file which contains the information iWeb needs.
Note that Apple have hidden the user’s Library folder to casual viewers in Lion, in an attempt to stop new Mac users messing their machines up. To access it, in the Finder go to the ‘Go’ menu and hold down the Option (Alt) key; the Library folder will appear as a choice.
Hopefully the file is backed up on Time Machine but if you can't find it you will have to reconstruct the site from scratch: you can use the text and images on the online version to save you having to type it all out again or find the original images, but iWeb can't import the online pages.
If you have trouble finding the domain file in Time Machine you can try using Find Any File to search your Time Machine hard drive for "Domain.sites2". FAF can search areas that Spotlight can't, like invisible folders, packages, etc.
When you find the latest domain file in the serarch results window just drag it from that window to the Desktop.