Hello zobel,
Thanks for reaching out to us!
Unfortunately there isn’t a single button click to cancel event and have users refunded and notified at the same time. There is a UserVoice page for this feature that you can vote on for our developers to consider for an upcoming release.
In the meantime, you could export the list of attendees and then upload their emails to an email blast service such as Mailchimp. You could then send out a notification en masse to all registrants. Refunds could also then be managed via the WooCommerce -> Orders section of your WordPress dashboard.
I’m sorry for the inconvenience but hopefully this will help to get you pointed in the right direction.
Thanks!