DIAGNOSIS This is usually caused by a full or improperly seated bag. If the bag is overfilled, it can press against the lid and create a vacuum seal that's tough to break. Or, the bag's paper collar isn't seated flat in its groove.
FIX First, try pressing down on the center of the lid while you press the release button. This can sometimes relieve pressure. If it opens, remove the old bag. Before installing a new one, ensure the bag compartment is clean and the rubber sealing gasket around the opening is free of debris. When putting in a new bag, make sure the stiff paper collar is fully inserted and flat in its channel all the way around.
IF THAT DIDN'T WORK Check the locking latch mechanism itself for debris or a broken plastic tab. Also, look at the hinge. A tiny bit of silicone lubricant on the hinge pin (not near the seal) can help if it's just stiff.
HEADS UP Replace bags before they look completely full. The general rule is 3/4 full. An overstuffed bag is the #1 cause of difficult lids and reduced suction.