DIAGNOSIS This is a safety feature. The valve is physically blocked from moving because there is still pressure inside the pot. Do not force it.
FIX You must release all pressure first. Move the entire pot to a cool burner. Do NOT run cold water over it (this can warp aluminum). Let it sit untouched for 20-30 minutes. The pressure will drop naturally as it cools. The valve will sink down on its own when it's safe. Then you can unlock and open the lid.
IF THAT DIDN'T WORK If the valve is mechanically jammed (e.g., by food debris) even after the pot is cool and depressurized, use a wooden chopstick or similar to very gently tap the side of the valve stem to dislodge any blockage. Do not use metal tools.
HEADS UP Never, ever try to force the lid open while the pressure valve is up. This is the cooker telling you it's still under pressure. Forcing it open is extremely dangerous.