Sometimes the sensor may become disconnected from the Quiet Punch iPhone app. Simply kill the Quiet Punch app (double tap the home button on your iPhone, your running applications will appear as windows, slide the Quiet Punch app up to end it). Restart the Quiet Punch app and you should receive the Connected message and should be good to go.
If this doesn't work please remove the battery from the sensor and put it back in. When you put it back in should light up green for a moment. If not light happens please let us know. If the light appears red then it is time for a new battery.