If you ever run into the case where you want to reset someone's Ticket Code, this is how you do it:
Lookup the patron's order via the magnifying glass in your top navigation.
Under "Order Details" next to the "Ticket Code" line item, click the Reset Code link and confirm. If you do not see the Reset Code link, this means it has not been used and will display as "Not Used".
Now your patrons can use the same Ticket Code to enter your event. If you'd like to resend them their order info, you can click the Resend Confirmation button at the top of their order's page.