DIAGNOSIS This is a vacuum lock. It happens when the cooker cools down with the lid locked and the pressure release valves seal shut, creating a slight vacuum inside that sucks the lid down.
FIX Don't force it. Gently try to rotate the lid back and forth. If it won't budge, put the cooker back on a cold burner. Turn the heat to the lowest setting for literally 30-60 seconds. You just want to warm the pot slightly to expand the air inside and break the vacuum. Remove it from heat and try to unlock. It should open easily.
IF THAT DIDN'T WORK Sometimes manually lifting the pressure regulator weight for a second to let air in can equalize the pressure. Do this carefully with a spoon, pointing the vent away from you. If it's still stuck, double-check that the float valve is truly down. If it's still slightly up, pressure might remain.
HEADS UP Forcing the lid open can damage the lugs and make the cooker unsafe. To prevent this in the future, always unlock and remove the lid as soon as it's safe to do so after a quick or natural pressure release—don't let it sit for hours locked and cold.