DIAGNOSIS This is almost always a float switch issue. The little plastic float inside the water tank that tells the machine the water level isn't moving freely or the sensor it connects to has gotten stuck.
FIX First, power the unit off. You need to get to that float. Carefully remove the water reservoir, pour out the water, and flip it over. Look for the small, clear plastic float in its vertical track. Move it up and down with your finger a dozen times to make sure it's not catching. If it's gummy, a quick clean with a cotton swab and some white vinegar will break up any mineral deposits. Dry it, refill the tank, and power it back on.
IF THAT DIDN'T WORK It's possible the float switch mechanism on the machine itself (where the tank sits) has a bit of debris or ice blocking it. With the unit off and unplugged, use a flashlight to look into the slot where the tank goes. You might see a small metal or plastic probe. Gently wipe it clean with a dry cloth. Plug it back in and try again.
HEADS UP Don't be tempted to just jiggle or smack the unit. The float assembly is simple but not super robust. Forcing it can break the little arm, and then you're looking at a real repair.