Ok. Let me first ask you, do you have any backups of your files - for example, do you use Time Machine or any other backup system?
Next, quit iPhoto. In the Finder, go to your Pictures folder. Is there a file there called iPhoto.Library? How large is it? (If it doesn't show the size, click once on it, click cmd-I to see the information). If it's a healthy size (ie, several GB), then that should be your library, and you can double-click it to launch iPhoto. (The application will remember where your library is next time, you can just launch iPhoto normally).
If that's not the right file, check your Trash. Is there anything in there?