Hi and welcome to Just Answer!
Sorry for your situation...
First of all the loss is only recognized if the property is sold.
If the value of the property is changed, but you keep the property - no gain or loss is recognized.
You will be able to deduct your loss only if that is a business or income producing property - for instance - a rental property.
If that is your personal property - you may not deduct the loss on your tax return.
You might be able to negotiate with the mortgage company to forgive part of your mortgage due to the market change. The amount of debt forgiven is reportable on 1099-C - http://www.irs.gov/pub/irs-pdf/f1099c.pdf
- generally is taxable, unless any exemption could be used
.The Mortgage Forgiveness Debt Relief Act of 2007 allows for federal tax
purposes an exclusion of income realized as a result of modification of the terms of the mortgage, or foreclosure on your principal residence
Sorry if you expected a different answer.
Let me know if you need any help or clarification.