Are you running Yosemite and have you recently updated to version 10.10.3? You may find that in so doing you have somehow switched to the new Photos app - if you have updated you will see the new app in your dock, it has an icon like a flower.
If that's not what's happened, you can find your iPhoto library in the following folder: Open the Finder and find your Home folder (the icon looks like a house). Open this folder and you'll see the Pictures folder.
If you can't find that folder search (press space bar and the cmd key) for Pictures. This is the folder where you will find the photo libraries. To access the contents, right click on the iPhoto Library icon and choose Show package contents. There's a folder called Masters, that will have all your images in it.
That's one way to know that you images are still there.
Now you know the location of the folder, you can remind iPhoto where to find it, when you open iPhoto hold down Alt and it will show you a dialogue box asking you which library to use. If the library isn't there then choose Other Library and locate the folder you found.
Hope that helps.