You owe them the money. When someone goes to these places they have to agree to the terms of the loan. Which means that they have the paperwork necessary to take you to court and sue you for the money. AND, if you gave them a check, they already know which back account to garnish to get their money fast.
They have to serve you either a summons and complaint, or a small claims form, depending on which court they use to collect the money. If they win the case, they can also add the filing costs, service of process and costs of collection to the amount of money they collect.
You can only be put in jail for a criminal action, this would be a civil action.
20 yrs as CA Paralegal