If the auction has ended and the winning bidder backs out then the
seller can if they want to, Offer you a second chance at your last bid.
You are not obligated to accept the second chance.
If the auction has not ended and the winning bidder retracts his/her bid
or if the seller cancels their bid and you are the next highest bidder then
you are high bidder and could win unless someone bids higher than you.