Calendar app icon is gone, says open when searched in the App Store but will not open

iphone 6s plus, calender app icon is not showing on any screen, i have tried to do a reset (holding home and power buttons until the Apple logo shows up) and I have tried to delete the app but only an app called “MobileCal” is showing up under settings, and it will not delete off my phone, there is not an app showing under setting just called just “calendar”