iphone calendar details and events have disappeared

Also check if your various calendars are enabled.

Tap "Calendars" in the bottom menu of your calendar. If there are no check marks in your calendars, tap once on the affected calendars and you should be able to see your appointments again.

Calendars are tied to email accounts. Have you deleted any email account you were using? Also, check in Settings > Your Name at the Top > iCloud > Calendars - make sure it's turned on there.