All entries in my iPhone calendar disappeared

"All entries in my iPhone calendar disappeared: I had added an appointment from my outlook calendar and checked that it showed up on my iPhone calendar. I went to enter something else a few minutes later, directly into my iPhone calendar and there are no events anywhere on the calendar. I'm panicking. I had a ton of appointments for my husband on there for medical appointments"

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Troubleshooting Missing Calendar Entries:

A. Visit this Site: If you Accidentally Deleted your Calendars, Bookmarks, or Contacts from iCloud - Apple Support

B. Enable Calendar in iCloud Settings:

Sign in to the same Apple ID on both iPhones, and then enable Calendar  in iCloud Settings. If these were synced in the iPhone's iCloud Settings, then all should presumably be visible.

On Both iPhones:

1. Click: Settings
2. Tap: your Name (top item)
3. Tap: iCloud
4. Tap: See All
5. Toggle On: Calendar (so that it is green) If already toggled on, toggle it off, and then toggle it back on.

C. Restore your iPhone from a Backup:

Have any backups of this iPhone prior to this occurrence? If so, restore your iPhone from one that has these appointments (the most recent backup, ideally).  Important: Back up you iPhone prior to performing this restoration, should anything go wrong from here on out.