Did you accept the booking request formally on Rover? The customer had 72 hours to pay and then it expires. So, if the customer hasn't paid, it isn't a booking and can't just drop off her dog. Message the client that you no longer have availability and she should be able to find another sitter on Rover. Then, call Rover to make sure that booking is closed.
In the future, you shouldn't accept other dogs when you have an open booking without getting that first one resolved either way. The person didn't respond to you, so you really don't have any obligation to keep a spot open for her dog.
Add: When a client sends you a booking, you have your M&G, then you "accept" the booking. The client has 72 hours to pay. If that time has passed without payment, there is no booking or potential booking and you are free to accept another client.
While the booking is "pending" (the 72 hours), someone else might contact you and want to go forward. If that happens, then you check with the first because you've already agreed and they have the 72 hrs. Let them know the spot will be gone.
As I said, you don't book while one is pending. If the first has expired, you are free to book whomever. If the first customer came to your door, you should refer them to Rover because you have no confirmation of the booking. That simple. Rover will need to find them another sitter.