Chances are the blower motor resistor has failed in your 2000 Chevrolet Silverado. The blower motor resistor is located under the right side of the dash near the blower motor. The electrical connector on the resistor comes from the blower switch and you can check the function of the switch there. If the switch tests OK the replacing the resistor would be in order. There is not much room to work when replacing the resistor but it can be done. Most times the resistor base is slotted on the firewall side so you don't have to remove those difficult mounting screws completely.