It sounds like you owe more than the property is worth in your market. Contact your lender; they likely don't want to take on the additional inventory. Most lenders have a lot of programs and options such as the possibility of a short sale. A short sale would give the lender something less than what you owe.
There is a lot to consider however, things like tax issues if the lender considers the short portion as income. They may also suggest a loan to you for the difference. Be sure you get the whole picture and understand it thoroughly. You may want to consult with an experienced agent and an attorney.
The agent can provide you with a complete understanding and picture of the market and the attorney can consult you on the legal and financial implications.
Connecticut Shoreline Property Specialist
Even if you owe more than the value, it may be worth it to sell it and bring $$ to the closing table. That expense might be a smaller price to pay than the limitations placed upon you as a result of a foreclosure. Lower the price and work out the difference w/ the lender. If you really lack the money to bring cash to the closing table, you can look into a possibility of a short sale.
There are a couple of options a loan modification if you want to keep the property or a short sale. You should call your current lenders loss mitigation department, explain your situation and fill out hardship or short sale paperwork. Then you will know what kind of payment terms are available with the loan modification and be able to make an informed decision on which option is best for you.
if it is not rented, and you still cant sell it, then refer to everyone else's answers of short selling or foreclosure.