I took the front panel off and exposed the mechanism. Inside the shield is a capacitor, so I guessed that the capacitor was bad. I ordered a new shield which contains a new capacitor, installed it, and the washer now works normally again. At first the pump motor made a few clicks before it started running, so I believe the drain pump had an obstruction of some kind inside it, like a bolus of sand, that caused the motor to seize temporarily and burned out the capacitor. It's possible the drain pump and the capacitor will have to be replaced together.