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There are two reasons for this problem. Either the iPhoto library is corrupted or the original photos are somehow deleted from the library. We can rebuild your iPhoto Library and see if that works or not.
Close iPhoto if it is open. Hold down the Command and Option keys on the keyboard and open iPhoto. Keep the keys held down until you are prompted to rebuild the library. A dialog will appear with rebuild options. Select all the options/checkboxes Click Rebuild. This may take a few minutes to complete.
After rebuilding, iPhoto will open. Check photos.
If it is still having issue after rebuilding, then we have to rebuild the iPhoto application using a third party application named iPhoto Library Manager. Download and install it from here (use the trial version) : http://www.fatcatsoftware.com/iplm/
Open iPhoto Library Manager. Select your iPhoto Library, click on Library menu in the top and select Rebuild Library. This will verify the library. Then click the Rebuild button on the right.
After rebuilding, open iPhoto. Check photos.
Please let me know if that didn't work, so we can continue troubleshooting. Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.