That means it can't find the original media files. Assuming you haven't been mucking around in the iPhoto Library manually, try this:
Open iPhoto while holding down the Command and Option keys. Click the boxes for all of the options in the window that pops up, then hit Enter or click the blue button. Let it run (if it's a large library, it could take a very long time). Then see if it responds.
If that doesn't work, I'd be looking at your Time Machine backups to recover the photos.
jW