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It is possible that you've accidentally deleted that application. Let's check if it is there or not. Go to Settings > General > Reset and tap Reset Home Screen Layout. If the Calendar app is there somewhere, then this will bring it to its default place.
If it didn't bring it back, that means you've deleted it somehow. You can redownload it. To redownload, open App Store. In App Store, just search "Calendar" (without quotes) and this will bring that app. Tap the cloud button or GET button next to it and it will be downloaded. Then you can open it and it will load your emails.
Another way to download is, open App Store, tap Purcahsed, tap All and you will see Calendar app there. Download it by tapping the cloud button.
Please let me know if that didn't work, so I will continue to help you until we fix the problem
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