Believe it or not, the most frequent reason for this is that the battery has gotten weak.We've found that if the battery has enough power to turn it on (the LED light comes on) but is a weak battery, the unit will NOT be able to detect the baby's heartbeat.So if the unit stops working but the light is still on, try replacing the battery with a NEW 9 volt brand name battery.However, if this hasn't resolved the problem then feel free to reach out to us at Customer Service.