That's normally an indication that iPhoto can't find that picture in the Library. Did you import that photo from Finder or an external drive/usb stick? Did you have the "Copy items to iPhoto Library" option in iPhoto Preferences>Advanced ticked?
If the answers to the above were "Yes" and "No" then if you've deleted the photo from Finder or you don't have the external drive/usb stick connected, then you have lost your photos.
If the above doesn't apply, then you might try restarting iPhoto with the command and option keys held down, which will bring up a repair dialogue. Check every option and give it time to do its thing. That might help, or it may not.