How to get the calendar back on my iPhone?

If you can't find Calendar on your home screen, you can try find it in the App Library.

On your home screen, swipe as far left as you can until you find App Library. Find the Calendar and drag it back to your Home Screen. You can also search for Calendar in Spotlight. When the icon appears, just like in the App Library, you can hold and drag it back to your Home Screen.

If the Calendar app is not in the App Library, you can download it again in the App Store.

If you have deleted an iCloud Calendar or some events, you may be able to save them again.

See this article: If you accidentally deleted your calendars, bookmarks, or contacts from iCloud - Apple Support

You can read more about the App Library here:

Organize the Home Screen and App Library on your iPhone - Apple Support

You can see how to use Spotlight/Search here:

Use Search on your iPhone, iPad, or iPod touch - Apple Support

Are you talking about the built-in Apple calendar? If so search for it, it is likely misplaced or hidden. Once you find it move it to your Home Screen.

If you are referring to a 3rd Party calendar, then go to the App Store and redownload it.