Why is my original iPhoto library gone?

I have been trying to recover my iphoto library for weeks but I can't seem to find it. Every time I open my iphoto library it asks me to choose a library but there are none on the list for me to choose from, so I've recently had to create a brand new library and I've lost all my photos and videos and I don't understand why. Ive also tried looking in my external macintosh hd for my original iphoto library but i can't seem to find it in there either. What else can i try???? I've lost over 3k photos :/