They stopped making the loan payment to you?

First you send a notice informing the borrower that the loan is delinquent and demand immediate payment of the amount in arrears. If that fails you may have to take them to court. If the loan is backed by a lien on an asset you can take the asset, but depending on where you live you may be required to obtain an order from the court before you are permitted to seize the asset. Otherwise you may get an order from the court for the other party to pay and then you'll have t try to find some assets and petition the court to attach those assets and then for an order allowing you to seize or garnish them. Or you can sell the debt to a collection agency that does this work all the time for a fraction of the face value. If you're not a lawyer, you'll probably do better selling the loan to the collection agency.