I lost mine a few months ago, replacement is quite easy.
I believe you get like 3 free replacements, but after that they might charge you. Just go down to your local SS office, I think you will have to take your ID & Birth certificate. My new card arrived in the mail, the whole process only took about 2 weeks for me.
I was right, check it out.
http://ssa-custhelp.ssa.gov/cgi-bin/ssa.cfg/php/enduser/std_adp.php?p_faqid=251&p_created=958404483&p_sid=a9O9MmQi&p_accessibility=0&p_redirect=&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9NSw1JnBfcHJvZHM9JnBfY2F0cz0xNiwzMSZwX3B2PSZwX2N2PTIuMzEmcF9zZWFyY2hfdHlwZT1hbnN3ZXJzLnNlYXJjaF9ubCZwX3BhZ2U9MQ**&p_li=&p_topview=1
"You can replace your card for free if it is lost or stolen.
To replace the lost Social Security card:
* Complete an Application For A Social Security Card (Form SS-5);
* Show us documents proving your identity; and
* Take (or mail) your completed application and documents to your local Social Security office or nearest card center.
All documents must be either originals or copies certified by the issuing agency. We cannot accept photocopies of notarized copies of documents. "
Documents You Need for a Social Security Card
http://www.socialsecurity.gov/ss5doc/
~Good Luck