The switch has a toggle mechanism that functions via a movable contact, cam, and a finger from the push button itself.
Remove the switch from the center panel (you'll need to pul the panel) by depressing the two retainer leaf springs at the top and bottom:
Then just pull the push button off the front of the switch.
These are the plunger and cam components of the toggle action:
Squirt some WD-40 into the cam component of the toggle action, and blow it out gently with canned or low pressure air. Do this a couple times then align the push button and press it back into place switch. Now work the switch a few times and listen for a subtle "click" on every other action, if you hear the click the toggle lock is working again.
Since it doesn't work now this is one of those "nothing to loose and a lot to gain" situations...