If an automatic payment has failed for one of your Exhibitor's orders, they will need to process the payment manually. They will automatically receive an email notification titled "Payment Failed" that looks like below and also contains their order information.
Once the exhibitor verifies their payment info is up-to-date, they can easily pay with the "Payment Failed Resubmit" button on the top right corner of their Order Summary box.
If it has been a while since the failed notice has been sent and they still have not completed payment you may want to send them a payment reminder. See our Help Center article on sending payment reminders here.