Apple Calendar disappeared. iCloud did not help

Do you use email accounts other than iCloud on the iPhone? For example, do you have Google, Microsoft, or other email accounts in use? I ask because nearly every email hosting provider provides not just email but also calendar, contacts, and notes hosting as well. It's possible that your default calendar was actually hosted somewhere other than iCloud. If this is the case, you may simply need to either add the account to the iPhone or enable calendars under that account's settings.

What was the hosting provider for the calendars that you do use for your schedule? Is that provider listed in Calendar's "Accounts" setting page? You may want to use a web browser to login to the account that you think that you were using for your calendar and see if the calendar data is where you think it should be. Once you you a web browser to find the calendar with your missing events then you can add that calendar to the Calendars app.