ALL PICTURES ARE COMPLETELY GONE FROM IPHOTO AND HARD DRIVE!

Hello,

My Mac Air memory disk was full because of the pictures storage and before I did any transfer to an external drive, they just completely vanished one day when I tried to open IPHOTO. There is a message asking me to select or create a new folder to open Iphoto. I don't know what went wrong but now I have 80gb of unused memory on my hard drive! Its like all my pictures are completely gone! Please help me recover them as they were picture of the 5 first years of my son! Thousands of them!