In most cases, the belt is loose. In this case, you'd better go to the repair shop to adjust the tension of the belt, because the belt will produce squeal when it slips. Once the belt slips, the belt will be further elongated due to the friction and high temperature, and the slip will be more serious and enter the vicious circle. So timely adjustment of belt tension is the key.
If you have driven nearly 20000 km without changing the belt before, you should change the belt now. Don't wait until the belt is broken to replace it.