If your order was canceled, there’s no need worry. After an order is submitted, sellers check their inventory to make sure they can fulfill the order. If they can fulfill the order, it's confirmed. If for some reason they can’t, it’s canceled and the pending charge is voided and will be reflected on your statement within a few business days.
This doesn’t happen very often. And when it does, it’s usually because a crazy amount of people are trying to buy tickets to the same event. Your best bet is usually to get right back on SeatGeek and purchase tickets from a different listing!