After installing the ios 11 update users are facing many troubles with existing working features of iOS10.One issue found is with lost calendar entries after upgrade to iOS 11. It was very surprising that the user found that all entries in calendar were deleted. To check and fix the issue make sure you try below listed options:
Fix 1: Go to settings, calendars, calendar sync and pressed All Events.
Settings>mail, contacts, calendar> sync (under calendar tab)> All Events
I hope this helps.
Fix 2: IF you find that your calendar events are all deleted.
So all your events will show up when you look at your calendar on your phone
Fix 3: Restore from to last backup in which you are sure that contains all calendar entry.