The Calendar app can not be deleted, so you won't find in in the App Store.
There are a few possibilities on why you can't find the icon.
One is that the app is inside a folder or on another home screen. If you pull down in the middle of the screen you should get a search field (iOS 7). Type in 'calendar' and it should show up as an App result. If it is in a folder the folder's name will be shown with the app's name. I detailed search of each home screen and folder may be required.
Whatever else happens, you can always launch the Calendar app from the search results. If you can use Siri, you can also tell Siri to Open Calendar.
Occasionally (rarely), if you've been moving icons around, the system will hiccup and hide the icon, or show it as a ghost. Restarting or reseting the iPad will usually clear this up. For a reset, hold down the power and home button until the Apple Logo appears. This can take 10 to 30 seconds. The iPad will reboot, and hopefully you'll be able to find the icon again.
Some of the built in apps can be hidden using Restrictions, but as afar as I know Calendar is not one of them. If you do play with Restrictions (in the General settings), do not forget the passcode you set. There is no easy way to reset it.
That's all I can think of at the moment.
Good luck.
