There are several reasons why the light is not on.
1. The system is in power saving mode. Press any key to display the last code on the LED display.
2. The device is without power. Make sure the cable is connected properly. Unplug the cable, wait a few seconds, and then plug it back in.
3. Make sure the cable to the electrode is connected properly.
4. If the device does not respond, the display may be faulty.