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Can you tell me the exact version of OS X you are currently running, please?
Click the Apple logo in the menu bar at the top left of the screen
Select "About This Mac"
In the window that opens, look for the "Version" information. Please let me know what it says.
Do you know which version of iPhoto you are using?
Click "iPhoto" in the menu bar at the top of the screen
Choose "About iPhoto"
What does it list for version?
Quit iPhoto if it is still open.
Hold down the Command and Option keys on the keyboard.
Keep the keys held down until you are prompted to rebuild the library.
A dialog will appear with rebuild options for you to select. I recommend checking all of them at this point since the reason for loss is unknown.
Click Rebuild to begin the rebuild process. This may take a few minutes to complete.