I have had this happen before where the website stated that only a few seats were remaining, and I got cut off in the booking process and tried to restart it afterwards In that case, the tickets came back after a while -- I don't recall exactly how long, but it was probably after a few hours.
Also immediate payment is only required for tickets booked through the website. For tickets booked by phone or through an agent, the seats can be held for a period of time before payment is required. Sometimes it's only a day or two, sometimes longer. Either way, you'll sometimes see cheaper tickets open up later on if the people holding those bookings let the payment period expire.