Again, what happens when you try? What specific app are you asking about?
What troubleshooting have you tried?
Are you saying that the stock "Contacts" app is not showing on your phone? Does it show up in the Spotlight Search? Did you accidentally put it in a folder? Did you delete it? Have you tried reinstalling it from the App Store?
Are you saying that the contacts IN the app are no longer there?
More details are needed.