Most system apps are protected and cannot be deleted unless you root the phone. More likely they were just removed from the home screen. Open the app drawer. I don't know what phone you have, but it's usually an icon across the bottom of the screen. May look like a group of small boxes, etc. Once open, scroll down till you see the apps you want back on the home screen. Tap and hold on the icon and the screen will switch back to the home screen. Release the icon where you want it to go on the screen. Repeat for other app icons.