loss of information

Was the system upgraded from an older to newer macOS?

If you had a good backup that used both Time Machine (software)

and Time Capsule (hardware) or an externally enclosed hard drive

where a duplicate backup of system & user files would be stored

either automatically or by use edict, this could allow full recovery.

Multiple backup strategies are recommended; including Clones.

{A quality clone with adequate suitable hardware, can be used to

restore a Mac from semi-failed state; the clone can run the Mac.}

Not sure if a 'data recovery' software could locate and retrieve

a specific user account's Calendar application. File corruption

or other file overwrite may have occurred. What major changes

could have occurred in the past year+ to cause this situation?

While there may be a place within your system/user account to look and hopefully

find some archive of iCal (or Calendar) information, the file may be corrupted or if

the system had somehow overwritten that, may be damaged or missing.

How to Clear the iCal Cache - wikiHow

How to Clear the iCal Cache. This wikiHow teaches you how to delete cached files

from your Mac's Calendar app. ... Open RAR Files on Mac OS X

https://www.wikihow.com/Clear-the-iCal-Cache

Another look at the macOS or OS X Calendar for sake of familiarity from here may
be helpful in general. Several ideas come to mind, and depending on which Mac

system your computer now has, there may be different means to attempt recovery.

Permissions and other details may apply. Here's a brief summary on that angle,

yet 'avoid changing permissions' as suggested in following article, unnecessarily.

How to Set File Permissions in Mac OS X | Macinstruct

How to Set File Permissions in Mac OS X. ... which are hidden beneath the Mac OS

X graphical user ... Assume you want to find a folder's current permissions and ...

macinstruct.com/node/415

The iCal or Calendar application has changed over time, yet remains similar.

Calendar (Apple) - Wikipedia

Calendar is a personal calendar app made by ... was the first calendar application

for Mac OS X to ... making Calendar easily configurable. Users can ...

https://en.wikipedia.org/wiki/Calendar_(Mac_OS)

For more info and exacting to specific need this Apple Support article may (or not)

be helpful to the extent possible within the range of Mac OS X or macOS you have.

Get help using iCloud Contacts, Calendars, or Reminders ...

Get help using iCloud Contacts, ... open iCloud for Windows. ... Open the Calendar

app and tap the Calendars tab. Make sure that All iCloud is selected.

https://support.apple.com/en-us/HT203521

Without knowing your Mac OS X vintage/version (signature line says Mavericks 10.9.5)

the more general info may or may not be informational or of use. There are other ways

to look for info such as Terminal utility gives Command line; & there's a learning curve.

All about calendar sharing | Macworld

All about calendar sharing ... (or an iCal backup file if you're using an earlier version

of the Mac OS). These files can't be imported by ... iOS Apps; Mac Apps;

https://www.macworld.com/article/2036317/all-about-calendar-sharing...

Calendar files use a specific file type, the above article suggests how to copy iCal

files and how to identify the icons. They have certain rules.

{Perhaps someone more familiar with how to access system files to use Find or search

in for other known locations for dated content otherwise lost, may see and reply here.}

Sorry to not be of much help given the iCal or Calendar application history is either

missing from your User account (Library) or corrupt. Confirm your macOS version.