Calendar disappeared after iOS update to 17.3 on iPhone 14
all calendar data on iPhone disappeared.
[Re-Titled by Moderator]
iPhone 14 Pro, iOS 17
Calendar events sync to email accounts. If you have recently removed an email account or lost access to one that might be an explanation of what caused this.
Or, it may have accidentally been turned off. Open the Calendar app and click on Calendars, and make sure your calendars are checked.