Which one had the broken gear? The throttle servo? If that's the case, then the motor is probably burned out on the servo.
If that isn't the case, then does the throttle servo work at all or is it just dead? If it doesn't work at all, check to make sure you plugged it in the correct direction. If it works but it's "weak", then again, it's probably a bad motor in the servo.
Try swapping in other servos and see if they do the same thing.