You can either contact the Social Security Administration at 1-800-772-1213 to request a replacement Social Security card, or register for a MY SSA account online at www.ssa.gov and provide documents as proof of identity. This can include a U.S. birth certificate or passport, hospital record of birth, or a driver’s license.
If this isn't possible for any reason, you'll need to visit a local Social Security Administration office to obtain a new card or request a new number.