Buyer's payment was declined after I accepted his/her offer. What next?

I just accepted an offer from a buyer, and the instant I accepted it, a message flashed very briefly on my screen. It said something about the payment being declined and that the item will remain available for purchase in my closet, but it disappeared before I could read the whole thing. Has this happened to anyone else? This buyer has made previous lower offers, and I've suspected that he/she might be a spammer. The declined payment only makes that suspicious feel more true. I can't find anything about this in PM's FAQs. Do they get a certain period of time to update their payment method? I can't cancel my accepted offer, can I? Appreciate anything you all can tell me. I've never had this happen.