The problem could range from a simple adjustment, air in the brake system or the most severe; total brake failure. Having a professional perform the necessary bleed, adjustments and inspection to determine the exact nature of the problem would be advised. Due to anti-lock brakes and the ever increasing complexity of brake systems, it is not a good idea to allow a "shade-tree" mechanic or yourself to risk damage to extremely expensive components. It is better to allow a certified technician to perform the task.