If you're experiencing issues with iPhoto not opening, a disappearing library or all your images just threw themselves off a cliff (I'm looking at you C.K.), I might have found a few solutions for you. The first is iPhoto Extractor, a freeware application (though I encourage donations) that can extract files out of your iPhoto library and organize them into proper folders that are ready for a re-import - weeding out all the extra database and thumbnail gunk you would otherwise run into doing this all by hand.

Before going this far to recover your iPhoto library, the app's author lists a few tricks and troubleshooting tips to see if less drastic measures can solve the problem. The first tip, which I didn't even realize is possible, is forcing iPhoto to try rebuilding its own library. Holding shift + option while starting up iPhoto 4 will present you with a dialog asking if you really want to rebuild. This *might* fix your issue, but your roll information will be lost. In iPhoto 5, hold option + command at the app's startup to be presented with a dialog offering a number of options including rebuilding thumbnails, rebuilding the entire database and recovering orphaned files.

For more iPhoto repairing goodness check out iPhoto Extractor and more tips on how to get your library back on track.