i let my moms boyfrinend get 3400 h e said he would pay it back but he didnt what can i do

If the person who owes you money signed a promissory note, you can enforce the promissory note by suing the person in court if the terms of the note were not followed and if the note was legally signed.  If there is no promissory note and you loaned the person the money within the time period in your state for enforcing oral contracts, then you can sue the person for the debt and attempt to collect the debt through the courts.  If the state or city where you live has a small claims court, you may be able to file a claim against the person in small claims court and the person will have to appear and answer your claim, and if the person does not appear, you will be awarded a judgment.  Then you will have to try to collect the judgment through legal means. There are many factors that can affect whether a debt can be collected, such as whether the money was a loan or a gift, whether there was a writing signed by the debtor acknowledging the debt, whether the debtor was competent to agree to the debt, how long the debt has been outstanding, and other factors. I am not licensed to practice law in the State of South Carolina, so this answer is intended to give you general legal information and is not intended to be legal advice.  You should contact an attorney in your state who is familiar with collection matters to obtain legal advice for your situation.