They must give you a chance to get your car back by paying off your debt. This is called a "right to redemption period." The length of this period depends on the state you live in, and can range from as few as 5 days to as many as 60 days.
After this period ends, the repo company can sell your car. However, they must provide you with written notice of the sale and the date and time it will occur. You also have the right to attend the sale and bid on your car.