My iPhotos application has disappeared with all my photos. HOw do I restore it?

OK (Note - screenshots and instructions below are from Catalina - they may look/be different on Yosemite)

hold down the option key while starting photos, and see if your old library is listed then. If it is, open it.

Were you using iCloud to sync your photos (If you were, and all your photos are in iCloud then an alternative to the suggestions below is simply to let Icloud sync to a new empty Library - it will take some time, possibly days and you need to keep photos open while it does it)

If not:

Open finder, and look in your pictures folder. What libraries are in there? (Normally somthing.photoslibrary). If there is more than one of those, you probably need the largest. Open it from the "other library" button above.

3 - If that doesn't help then you are going to need to do a search in spotlight.

In finder, in the search bar, type "photoslibrary". Make sure it is searching "This Mac"

What does it find?

Do you mean iPhoto (discontinued some years ago?

Or do you mean the photos app.

What do you mean by dissappeared. You really need to give much more detail about your problem, and about your system.