If you store your iphoto photos on an external hard drive and remove the hard drive sometimes iPhoto gets a little confused. When you next start iPhoto it shows you an empty library. Don't panic! Your photos should still be fine on the external drive it's just iPhoto doesn't know where to find them.

The traditional solution is to delete the iPhoto folder in HD/users/username/pictures and then when you start iPhoto it asks where to find the library. But this does not work in this case. Instead it seems to create another folder in HD/Volumes/Username/iPhoto which is weird. If you can find this and delete it then this solution will work.

Other wise just hold down Option before you click on the iPhoto icon and it will ask you where the library is - reselect the external HD folder and bingo. She's a beauty.

To rebuild your iPhoto library:
Hold down cmd and option when you launch iPhoto and attempt to rebuild your library.

To only re-find the iPhoto library.
Hold down option only and tell iPhoto where the library is.

Hope that helps someone - it took me ages to find the solution.