DIAGNOSIS This is almost always caused by a small piece of food or starchy residue that got sucked up and is stuck under the float valve, holding it up. It creates a vacuum seal.
FIX 1) First, ensure all pressure is released (the weight isn't rocking and no steam is coming out). 2) Try gently tapping the top of the float valve with the handle of a wooden spoon. Sometimes the vibration is enough to dislodge the debris. 3) If that fails, take a thin, non-metallic tool like a bamboo skewer or a toothpick and very carefully slide it under the edge of the float valve's silicone cap to break the vacuum. You should hear a slight hiss and the valve will drop.
IF THAT DIDN'T WORK You may need to remove the anti-clog shield (see the other cleaning question) to get better access to the base of the float valve. Clean the area thoroughly.
HEADS UP Do not use a metal knife or screwdriver to pry the valve up. You can easily damage the silicone seal or scratch the sealing surface, which will cause leaks later. Gentle persuasion with soft tools is the way.