If an event is cancelled or postponed, first you'll want to make sure the event is inactive and ticket sales are shut off. Next, you should work with the organization putting on the event to see how they want to handle this.
In most cases, they have a refund policy that you can refer to, but there are certain circumstances (i.e. weather) where they may accommodate customers differently.
Tip: If this is a large event, you'll want to make sure your Transaction Limits with Forte will cover all the refunds. If not, you can request a limit increase, or simply split these refunds up across days.