For clarity, once you've dialed in the correct combination, how did you open the cabinet? Was it the thumb latch below the combination dial? Have you tried dialing the combination and using the thumb latch since the button was pushed in? Given that, have you tried pressing the drawer in while operating the thumb latch? Again, just blue skying a few ideas. Were this a computer I would try pulling the power plug, but...
Anything to engage the internal latching mechanism, or rattle it. I'd suggest rocking the cabinet while operating the thumb latch as well.