It is very common for buyers to become discouraged during the lengthy waiting time while dealing with a lender in a short sale. They often bolt and move on to other properties. The standard answer in your dilemma is that you should contact a real estate attorney. In the long run you may end up spending money you will never recover. It may be best to move on and find another buyer.
You already received good answers. It seems that the buyers did back out of the deal and your agent can look for other buyers now. If the first buyer is still interested you will know shortly. Once you get out from under this house, if you need assistance with credit repair I will be happy to assist you. Good luck and I hope you are able to push through this short sale soon.
The Credit Restoration Expert