If a widget disappeared from your home screen, then add the widget by touching & holding an empty spot on your home screen.
If an app disappeared from your home screen, the shortcut for opening the app was removed and you just need to add a shortcut.
If an app disappeared from your list of all apps, you need to reinstall it.
After each step, check to see if the issue is fixed:
Turn your phone off and then on again.
Press and hold the Power button, then touch Power off.
Contact the app’s developer.
If apps are disabled because you’re in safe mode, you can’t use them until you exit safe mode.
To exit safe mode, turn your phone off and then on again.
After each step, check to see if the issue is fixed:
Go to the developer’s website for support.