"Less than 24 hours" is NOT an unreasonable time to not hear back--in fact, it's perfectly reasonable for the seller to not have responded yet. Your current offer is still "on the table" and they can accept it. You'd have to first withdraw that offer: you would need to send them something in writing (send it some way you can prove delivery) giving stating that they have soome short, but reasonable, time to accept the offer or it will be withdrawn (e.g. if you get them the writing early am, by close of business that day. After you have withdrawn your current offer, you could then replace with another offer. In the future, if you need a quick response, put a time limit into the offer when you transmit it.