DIAGNOSIS The water level sensor is likely blocked by scale or mineral deposits, so it can't detect the water even when the tank is full.
FIX Descale the unit immediately. Unplug it, drain all water, and fill the reservoir with a 50/50 mix of white vinegar and water. Let it sit for 15-20 minutes, then run a cleaning cycle (press and hold the Power button for 3 seconds until it beeps). Let it run through a full cycle, then drain and rinse thoroughly with fresh water twice.
IF THAT DIDN'T WORK Check that the water inlet screen (inside the reservoir where water flows into the machine) isn't clogged with debris. Unplug the unit, use a small brush or toothpick to gently clear any visible gunk.
HEADS UP This is almost always a maintenance issue, not a broken part. If you have hard water, you'll need to do this vinegar clean every 2-3 weeks to prevent it from happening again.