iPhoto isn't working on my 2011 MacBook Pro with OS X Lion
You need to reinstall:

First: make a backup. Any kind of back up. Always have an up-to-date back up of your iPhoto Library. 

To re-install iPhoto
 
1. Put the iPhoto.app in the trash (Drag it from your Applications Folder to the trash)
 
2: Trash the Receipts: In the Finder use the Go menu and select Go To Folder. In the resulting window type

/private/var/db/receipts
 
A Finder Window will open at that location and you can remove files that you see with iPhoto on the name.
 
Re-install.
 
As you have a recent make you do this via the Purchases List at the App Store.