Hi Caroline,
Eventbrite Tickets requires The Events Calendar, but it does not require Event Tickets or Event Tickets Plus. Can you double check that you have The Events Calendar installed and activated? If so, can you please share your system info with me?
Event Tickets and Event Tickets Plus do not require The Events Calendar to function (you can add tickets to any post type, not just events), so it is possible that you were selling tickets without The Events Calendar previously.
Please let me know if activating The Events Calendar gets things working again!
Thanks,
Jennifer