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There are mainly two reasons for this problem. Someone has accessed your iPhone and removed them or there was a software update that caused this problem. But you can redownload them from App Store. Open App Store application. Search the name of the app (like Facebook, Google) and this will bring the apps on main screen. Then tap the Cloud or Get button next to the Apps to download them to your iPhone.
Then go to Settings > Touch ID and Password ***** change the passcode. So one will have access to your iPhone.
If you have any questions or would like any additional information, please don't hesitate ask.
Make sure your iPhone is passcode protected. Go to Settings > Touch ID and Passcode (or Passcode) and change the passcode or turn on passcode there. So one will have access to your iPhone.