you mean its showing 0 watts on the bike display, or zero on your garmin/other bike computer?
Thats odd, sounds like a fault, it should display your power , changing the cranks shouldnt make any difference, the sensor for your power is inside the motor, and not part of the cranks.
If its not detecting your power at all then you'd expect the motor to throw an error because it uses your power input to determine how much power the motor should add.
Time to take it back to the shop if you're certain your on the correct screen on the display (I cant see how you might not be on the correct screen , but anyway.....)