If your roaster is on but the cooling sweep won't move when you flip the switch, here are a few things to check.
1. Check that the breakers in the electrical box are not tripped. An overloaded circuit could have caused one of the breakers to trip and would prevent the cooling sweep from engaging.
2. Check the reset in the bottom right corner of the breaker panel. To reset, turn the knob all the way to the left and then back to the "ON" position. Test the cooling sweep after performing this reset to see if that allows it to move again.