I can't find my calendar on my iPhone

First, check if Calendar is enabled in iCloud, go to:

Settings → Apple ID → iCloud → check that Calendar is enabled on the list.

If Calendar is enabled, open your Calendar app. Tap Calendars at the bottom, in the middle. Can you see your appointment calendar on the list, if so, tap on it so that it gets a check mark on the left side.

If your appointment calendar is missing from the list, you can try to recover it from https://icloud.com

You can see how to recover lost calendars here:

If you accidentally deleted your calendars, bookmarks, or contacts from iCloud - Apple Support

First try a Forced Restart:

Force restart iPhone - Apple Support

Have you tried searching for it? The calendar did not delete itself.