So you need to equalize the pressure:
1) Try to warm it up. The air inside will expand hopefully enough to let you open. How you do this is going to depend on what resources you have available and of course you don't want to damage the camera.
2) Even a tiny needle hole (heat a needle with a match) will be enough to equilibrate, but then you are no longer totally waterproof. But maybe you could reseal the hole?
3) Push continuously in one spot near where it closes; if you can vent it, you're home free.
Good luck