iPhoto is on my dock, but won't open

iPhoto is on my MacBook Pro's dock, but it will not open.

When I click it a "?" appears on it, but that's it. Also the iPhoto icon is no longer in the Apps folder.

I didn't, at least not knowingly, delete the app. I don't have the disk needed to reinstall it on my mac. Is there any other way to get the app back without having to pay for it? If it was accidentally deleted or if something else caused the problem, it doesn't seem right to have to pay for it...again😢. Help!