If an employer refuses to pay, the employee can file a report with a government agency. The department of labor is usually responsible for handling such disputes, and may have a department that specifically focuses on wages owed. The agency can review the case and may decide to take it to court. It pays the court costs for the suit and works with the court to receive an order. This is more common in cases involving multiple employees or particularly egregious examples of back pay owed.
It is also possible to take the employer to court privately. Employees can file suit under the labor laws of their region to demand pay and receive a court order. This will require some out-of-pocket expenses, and employees may want to balance how much they expect to receive against the costs of filing suit in court. Employers tend to have deeper pockets and may be able to fight the case long enough for the employee's funds to run out, which is something to consider before filing suit.
With a court order to collect back pay in hand, the employee can demand payment from the employer. If the employer refuses to comply, the court can compel the employer to surrender the funds. Employers who feel the order is unjust can potentially file a countersuit to argue their side.