Your lender may modify your loan if you have an adjustable rate mortgage or if you are several months behind on your mortgage. Call and ask to speak to your lender's loan modification or loss mitigation department. The lender may accept partial payments for a few months (though you may have to agree to make up the difference later), accept a late payment, or agree to modify the terms of your loan.
If your lender doesn't want to work with you, contact a HUD-approved housing counselor for help negotiating with your lender. HUD-approved housing counselors work for free and are well-versed in the various foreclosure-prevention programs that are available. To find a counselor, contact the Homeownership Preservation Foundation at www.995hope.org or 888-995-HOPE (888-995-4673).
There are several plans offered by the federal government to help homeowners avoid foreclosures as part of the Making Home Affordable program. The two biggies are the Home Affordable Refinance Program (HARP), under which you may be able to refinance your current mortgage into a 15- or 30-year fixed-rate mortgage, and the Home Affordable Modification Program (HAMP), which may allow you to lower your monthly payment on your first mortgage to 31% of your gross income.