Calendar Evens Missing / Disappear / Lost iPhone iPad

If you are unable to locate Calendar events

Calendars are part of an email account (Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other) See the pic below

The settings for the Calendar app might get reset when you upgrade or restore or reset your iDevice. That reset triggers a change in the way calendar events are synced to the stock Calendar app. And this might prevent older events from syncing to the calendar app. To fix this, here’s what you need to do:

They are usually part of an email account such as given in the image below

If so. Please watch the GIF change the sync days

Keep your Calendar up to date with iCloud - Apple Support

https://support.apple.com/en-in/HT202337