score:6
Don't give up hope just yet: it's possible that your previous session is still holding on to the last seat. You can't access the previous session anymore, but the hold won't last more than 30-60 minutes, so try again and you might find it again.
Upvote:7
Your reservation only becomes valid after the final booking step - anything entered in between would be discarded if you don't finish the payment process. So unfortunately the flight in question seems to be booked out (not surprising given that it's so close to Christmas) and you're out of luck.