As far as I remember, there is no locking mechanism on the lid. The unit simply detects it going up or down to control the mechanism. If this is a unit that has been in storage or unused, it could have dried damping grease at the two hinges. I would first try using a blow dryer at low setting at the lids back hinge points to soften it. Then try lifting it by both front corners so the upwards pressure is equal.
