DIAGNOSIS This is residual vacuum pressure. You likely opened the pressure valve before the float valve dropped, trapping a vacuum.
FIX Do not force it. Take the entire unit to your kitchen sink. Slowly run warm (not hot) water over the lid and sides of the pot for 30-45 seconds. The warmth will gently equalize the pressure inside. You should then hear a faint hiss or pop. The lid should now turn open normally.
IF THAT DIDN'T WORK Ensure the pressure regulator is set to 'VENT'. Try tapping the float valve gently with a wooden spoon to see if it's stuck up. Wait longer. Sometimes a full cool-down of several hours is needed.
HEADS UP Never use tools to pry the lid. You can warp the sealing edge. The warm water trick is the safe, manufacturer-recommended method.