Howdy holmfirthevents,
That is pretty unusual. I am not able to find anyone past or present that has experienced the same thing. I am concerned that something highly unusual happened, perhaps a PHP timeout or memory allocation error, during the upgrade.
If you are confident that the only thing you lost is your organizers, you could make a backup of today, then revert to the old site. Go to WP Admin > Tools > Export, and export your Organizers & Venues. Then restores today’s backup and import your lost data (Which requires downloading the official WP Import plugin). That said, you would want to make doubly sure that you did not lose any other data, and that your site is functioning properly. It also could not hurt to manually look through your database and make sure nothing looks corrupter.
Does that make sense? Will that work for you? Please let me know. Cheers!
– Brook