When you re-opened iPhoto, was the whole photo album empty, or were there just some changes you had made in that session missing?
If it's the former, then there is a bug in iPhoto that sometimes causes it to launch and think there is no photo album there. You can fix this by closing iPhoto, and holding it while holding down the Option key. A window will pop up, and say that iPhoto needs an album to contine. Point it to yours (should be in your pictures directory), and it should return to normal.
If it's the latter, iPhoto doesn't database changes like, say, Aperture does, or Photoshop does when you're working in Photoshop native format. What iPhoto does instead is makes a copy of the file as the original, and then permanently modifies one copy while holding the other one. So I'm not sure how this could have happened. It isn't like iPhoto periodically saves things. It changes the file from the very beginning, pretty much. So I don't think there're temporary files of the sort you are looking for. If you look inside a folder in your iPhoto library, for a specific date, you'll see an "originals" folder, and then the images in the root of that date directory will be the modified ones....
You might also be able to benefit from rebuilding your library. You do this by launching iPhoto while holding Option-Apple.
Please say if any of this helps?
