You need to speak with a real estate attorney right away. If the owner took your money on a valid lease/option or rent to own contract, then stopped making mortgage payments, that may be considered fraud. There are so many questions that need to be answered in order to make sure you have a valid contract and also to determine what, exactly, the situation is regarding the home. Does the owner have equity in the home or are they 'under water' owing more than the home is worth sparking a short sale or foreclosure situation?. Were you represented by a real estate professional in this transaction? Are you still able to purchase the home and receive the promised credit toward the purchase price and/or down payment? Does the owner have other assets that could be used to compensate you for any losses? Is there insurance coverage that might compensate you in this situation?
A good lawyer will help you wade through the morass and determine how best to preserve your ownership rights in the property.