Sounds like iPhoto does not know where the originals images are. Are they on an external drive that is not currently connected to your computer?
It's possible you have disabled the option to copy the files into the library on import, so while iPhoto has created thumbnails of the images, it cannot display/edit the full image without the original drive/device (e.g., your camera) connected to the computer. Another possibility is that your library has become corrupted. You should still be able to retrieve your photos when you open the contents of the library package, but you will most likely lose all you customization (keywords faces, places, events).