You either settle out of court or you end up in court. Given that the debt is probably well documented, the court will issue a judgment against you, perhaps including the court costs (usually you have agreed to this in the contract). Then the creditor will give you a certain time to pay off the money, or they will garnish your wages, foreclose or seize assets to be sold.
You respond and defend youself.
If you have no defense, you pay.
If you don't or can't pay, they can get a judgment and may seize your property assets.