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