(1) If this is a short sale, the final decision rests with the lender. MLS rules differ from area to area. Some mandate that the status of the sale should change to pending once the seller accepts an offer. Others say change it to pending only when the lender accepts the offer.
(2) Was the offer accepted contingent on the buyer selling his own property? If so, the listing may continue to be offered as active until the buyer sells his own property so that he can move forward on the one he's buying.
(3) It could also be mere oversight on the part of the listing agent who has neglected to change the status on the MLS, or on the webmaster who hasn't updated the website
(4) The previous buyer may have backed out of the agreement. And if there wasn't a back-up offer, it will continue to be offered as active.
At any rate, ask your realtor/buyer's agent to call the listing agent and verify that the property you wrote an offer on is indeed no longer actively for sale. If it's still active, here's your chance to submit a new offer and give it your best shot.