What version of iPhoto and system are you running? How did you backup your library? If it's not Time Machine then follow TD's suggestion.
Also if you're getting that message when you try to open the restored library try the Terminal commands set forth in the Resolution paragraph of this Apple document: iPhoto 6: "Unable to upgrade this photo library" message
IPhoto 11 version 9.4.2. I did a back up in time machine to an external hard.
I have since tried following Old Toad's directions i.e.; using the
Apple document: iPhoto 6: "Unable to upgrade this photo library" message
I tried to restore iPhoto library again and got the same message.