DIAGNOSIS The float switch in the water tank—that little white plastic piece that moves up and down—is likely stuck or the sensor contacts are coated with mineral scale. The machine can't tell there's water, so it won't operate.
FIX 1) Remove the water tank. 2) Look at the bottom where it connects to the machine. You'll see the float assembly. Gently push up and down on the float with your finger to make sure it moves freely. 3) If it's sticky, clean it. You can soak the bottom of the tank (or just the float area) in a mix of warm water and a little white vinegar for 20 minutes to dissolve any limescale. Scrub gently with a soft brush. 4) Rinse thoroughly and dry the contacts on both the tank and the machine's base with a clean cloth. Reinstall the tank firmly.
IF THAT DIDN'T WORK The sensor in the machine's base might be faulty. Try unplugging the machine for 10 minutes to reset the electronics. If the problem comes back immediately, there might be a wiring issue or a failed sensor that needs professional attention.
HEADS UP Always use filtered or softened water if you can. Hard water minerals are the main cause of this float sticking. If you see a white crusty buildup anywhere in the tank, that's scale, and it's time for a descale cycle too.