G'day, and welcome to the forums.
What happens when you hold down ⌘Command & ⌥Option when you click on the iPhoto Icon to open it ?? Do you get a Window pop up with Options ?? If so, try the top Repair 1st and then try and open it, if not, do the same and work your way down the list doing Repair Database and then Rebuild Database and forget the Thumbnails option.
Do all those in turn trying to open iPhoto after each one, and if the problem persists return here and we will look for another option.
When you come back, let us know what Mac you have and what OS you are running as well as what .v of iPhoto please
Cheers