DIAGNOSIS The floating valve (that little silver pin in the lid) is likely jammed in the 'up' or sealed position because some food gunk got inside, or the silicone seal around it is misaligned.
FIX 1) Don't force it. Unplug the unit. 2) Push the 'Steam Release' handle to the Venting position and hold it there for a minute to make sure all pressure is gone. 3) Look at the silver float valve on top of the lid. Wiggle it gently up and down with your fingers. Sometimes it just needs a nudge to drop. 4) If it's still stuck, remove the inner pot, turn the main unit upside down, and gently tap the lid against a folded towel on your countertop. The jiggle can dislodge a tiny piece of food.
IF THAT DIDN'T WORK The silicone sealing ring might be pinched. Remove the lid's inner parts: pull out the sealing ring, then pop off the float valve and its silicone cap by pushing up from underneath. Wash all parts in warm soapy water, dry them, and reassemble carefully, making sure the sealing ring is seated evenly in its groove.
HEADS UP Always make sure the float valve moves freely before you start cooking. A quick check with your finger prevents this headache.