I just lost my calendar and I need this .
Don't do the 18.0 update
Read here:
https://www.reddit.com/r/iOSBeta/comments/1di0gub/ios_18_db1_empty_calendar/
I powered off my phone and it fixed the problem!
Hope that works for you as well.
If not working, may have a look here: https://www.youtube.com/watch?v=h0VPocWXc9g