You can't force them to buy the house. But you can make them pay a financial penalty for their breach of the contract. The first thing you do is have your attorney send a 'time is of the essence' letter immediately when the date for closing arrives and they do not show. If that doesn't bring them to closing, then you sue them for specific performance. You can sue for any expenses of the sale. You can't sue for replacement of things you sold. You can sue for the real estate commission that YOU owe to the listing broker. YOU owe that commission, the real estate professionals involved did their job and earned the commission. The buyer does not pay the commission but they can be sued to make you whole. In addition, should you put the property up for sale and end up selling it for less the second time, you can sue the buyer for that loss. If their breach of contract caused any purchase you were making to fail and you were sued, in turn, by that seller, you could sue your buyer for what cost you.
Recovering all of those damages is another story. You need to get your attorney involved in this immediately.