DIAGNOSIS: The float sensor in the water reservoir is stuck in the 'low' position. This is usually due to mineral scale or the float catching on the side of the tank.
FIX: Remove the water reservoir. Shake it. You should hear the plastic float inside moving freely. If not, fill the tank with a warm vinegar solution and let it soak for 30 minutes to loosen scale, then shake vigorously.
IF THAT DIDN'T WORK: The electrical contacts on the reservoir or the base where it sits might be damp or corroded. Unplug the machine. Dry both the bottom of the reservoir and the contact pins in the machine's base with a paper towel.
HEADS UP: Only use the manufacturer-provided reservoir. The water level sensor is calibrated for its specific shape and float placement. A different container won't work.