If you're using iPhoto 11 then check the system trash.
Earlier versions: you'll need an app like File Salvage - you can download a free trial and it will scan the disk for you and tell you with might be recoverable. Actual recovery will require that you purchase the app. There may be many files with the same or similar names. Always recover the largest file size.
There are other such apps. Search on Macupdate or the App Store
Of course, if you have a back up then that's the best solution.