Sue some one

Any lawyer who handles civil litigation should be able to handle your case but, depending on the amount of the loan, you may want to engage an attorney who specializes in collection matters, who may be willing to take your case on a contingency.  From the very limited facts  you've set forth, you seem like  you have a good case (assuming that the breach occurred within the statute of limitations), but whether you can collect also depends on whether the debtor has any money to satisfy any judgment.