Usually, that app gets 'linked' to the Google account you are using to connect with the play store. This prevents you from having to purchase it again if for example you got a new device, or you accidentally uninstalled the app.
So if you're still using the same account that you used when you purchased the app, then you should be able to just go back to the play store and reinstall it. If it says it's already installed on your device, then you already have it, in which you need to look in to "All applications", and find it there.