I am having the same trouble.  All calendar events are gone.

I've split this into its own topic.

Can you please clarify what was done in the ACP or on the front-end, or what else may have happened, right before the events went missing?

This is a known issue unfortunately, as mentioned in the ticket  you have open. A bug report for this has been submitted

Please could you confirm its actually birthdays you are referring to on this one?

This has been resolved in the latest 4.6.8 release. Please upgrade to get the latest release, and let us know if you see any further issues.