DIAGNOSIS The drive hub assembly has failed. The plastic drive hub connects the motor shaft to the belt pulley. It's a known wear item.
FIX You need part #41A4276, the drive hub kit. Unplug the opener. Remove the three screws securing the belt cover. You'll see the white plastic hub on the motor shaft. Remove the set screw, slide off the old hub, and install the new one. Tighten the set screw firmly.
IF THAT DIDN'T WORK The belt itself may be stripped. Inspect the teeth on the belt where it engages the pulley. If they are sheared off, replace the belt (part #41A4274).
HEADS UP The door is disconnected during this repair. Manually lift the door and secure it with a clamp on the track before starting.