how to restore calendar app

See this Apple support article—>Remove built-in Apple apps from the Home screen on your iOS 10 device or Apple Watch - Apple Support. Note, however, that contacts are not stored in the app itself; the app accesses database of contacts, and those databases are associated with email accounts you have on the phone.

Use the Search function (drag down from the middle of the screen) and search for “Calendar”. If the app appears in the search it is somewhere on your phone. You can look for it on other pages of the home screen, or in folders. If it is in a folder the Search will have the name of folder next to the app.

Hi. This happened to me as well when I transferred from my iPhone 6s to X. This is what worked for me:

1. Sign in to iCloud.com.
2. Click the Settings button
3. Scroll down and (under Advanced) click Restore Calendars and Reminders
4. Click Restore next to the date before you deleted your calendars. 
5. Click Restore again to confirm. You should get an email confirmation

Hope this works for you too.

Good point! I had forgotten that. Thanks for jumping in.

Thank you!!

Glad I could help! :-)