If you lose your social security card, you can replace it by filling out the SS-5 form, which you can download from the Social Security web site. You’ll need to supply proof of your identity and US citizenship via mail or in person at your local Social Security office. For more information, visit http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/251.
If you've lost your social security card, you must immediately go to your local social security office for a replacement. You will need documents proving citizenship, age and identity. For more information, visit: http://ssa.gov/ssnumber/ss5doc.htm.