While you clearly have an economic loss, tax rules prevent you from deducting the decline in value from your $120,000 purchase price to the value at the time you converted the condo to rental property. What’s more, depending on the amount of depreciation claimed on the condo from the time of conversion, you may actually have a tax gain (even though you have a financial loss). Talk with a tax pro to run the numbers in your situation.
On Form 1040X, you may file an amended return within a three-year period to claim a refund or correct a mistake made on an original or previously amended return.