All iCloud calendar events have disappeared

My partner uses Calendar on her M1 MacBook Air running OS X Sequoia. She has six calendars stored on iCloud.

Yesterday, all the events disappeared from her calendars. When I investigated, I found that Calendar had reverted to showing the local calendar, rather than the iCloud calendars. I re-enabled the iCloud account, and the iCloud calendars reappeared in the Calendar sidebar, but the events remained missing.

When she logged in to iCloud.com in Safari, the copies of the calendars there were also empty.

Data Restore offers the option to restore the calendars, but the only archive available is from 7:59PM yesterday, after the problem first occurred.

I checked Calendar access under Privacy & Security, but I don't think that's the problem. Calendar doesn't appear in the list of authorized applications on her machine, but it doesn't appear in the list on mine either, and Calendar works fine on my machine. I assume that Calendar automatically has the right to access calendars, and doesn't need to be granted special permissions through Privacy & Security.

I don't believe that she did anything to cause the events to be deleted (it would be very hard to delete hundreds of events by accident), but her machine may have had issues with running out of disk space around the time that the problem occurred.

The machine is regularly backed-up using Time Machine (and other backup solutions), so backups are theoretically available. I don't know if that will help when the calendars are stored in the cloud, rather than locally. Restoring the contents of ~/Library/Calendars did not seem to help.

Is there any way to recover the lost events?