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 properly connected. Disconnect the cable, wait a few seconds and reconnect it.
3. Make sure the cable to the electrode is properly connected.
4. If the device is not responding, the display may be malfunctioning.