Hello and sorry to hear about the troubles,
The upgrade in no way should have altered/deleted the existing events. If you can remember – what version were you updating from? Additionally, can you confirm the information is no longer in the database?
One way I can think of to possibly go about recovering the lost data would be to contact your hosting company. They usually have weekly backups available for free or a small fee.
Again, sorry to hear about the potential loss of data. I don’t mean to point out the obvious, but we always recommend making backups prior to updating themes/plugins.
Let me know if any of this information helps and best of luck getting it all addressed. Have a good day 🙂