I filed bankruptcy and my job has $1300 from a garnishment. how can i get this money back?

I'm not sure what you mean when you say your "job" has $1,300 from a garnishment.  Do you mean your employer garnished funds from you?  Do you owe your employer money?  Or are you saying a creditor got a judgment against you and obtained a court wage garnishment order and served that on your employer who then sent the $1,300 to the creditor?   You need to find out who has the money and send them the bankruptcy notice and demand it back.  Depending on when it was taken, you may or may not have problems getting it returned.