I don't know why but my iPhoto on my dock just has a picture of some pencils and an ø through everything and it was the same thing when I looked at it in the applications. When I try to open it, it says that it is probably damaged but it worked just last week. Btw my MacBook Pro was shipped with OS X Lion so I don't have any startup disks or anything like that and I never set up Time Machine and I still don't know how to set it up, so anything involving those 2 are out of the question.
You need to reinstall:
First: make a backup. Any kind of back up. Always have an up-to-date back up of your iPhoto Library.
To re-install iPhoto
1. Put the iPhoto.app in the trash (Drag it from your Applications Folder to the trash)
2: Trash the Receipts: In the Finder use the Go menu and select Go To Folder. In the resulting window type
A Finder Window will open at that location and you can remove files that you see with iPhoto on the name.
As you have a recent make you do this via the Purchases List at the App Store.