If you received an email stating that your tickets are no longer available, there’s no need to worry.
After an order is submitted, sellers check their inventory to make sure they can fulfill the order. If the tickets for your order are available, it's confirmed. If for some reason they aren't available, then your order is canceled and the pending charge is voided. You'll see the money reflected on your statement within 3-5 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!