DIAGNOSIS The float sensor that detects water level is either stuck or gunked up, so the machine thinks it's empty even when it's not.
FIX Unplug the machine. Take out the water reservoir (if removable) or just slosh the water around vigorously inside the main tank to dislodge the float. Often it's a small plastic bobber on a hinge. Manually move it up and down a few times with your finger to free it. Dry everything off, plug it back in, and see if the light turns off.
IF THAT DIDN'T WORK The sensor itself might have failed. Try a full reset: unplug, empty all water, leave it unplugged for 30 minutes, then refill and restart. If the light's still on, the sensor circuit is likely faulty and needs service.
HEADS UP Using water with high mineral content can speed up this problem by coating the float with scale. Consider using filtered or distilled water to prolong the sensor's life.