What has happened to my photos in I Photo events

All of my "events" photos in i photo can no longer be seen unless i open them in slideshow format. Does anyone know how I can recover them. The "events" folders are still there and have the titles of the folders on them, but there is no key photo and when I click on the folder no pohotos show up, unless i open them in slideshow format. Very frustrating.

Have you moved or otherwise messed with the iPhoto Library?