Someone kidnapped you and made a rectangular concrete block with a space in the middle. They put you in the space and cover it up. Now do not think that you would run out of oxygen because you find out that there's a tube running to your nose giving you oxygen and there a tube connected to your throat that feeds you.
WHAT WOLD YOU DO?
Honey, I do not know what I would do, but I definitely know what you shouldn't do - watch too many horror movies.
Well the first thing to do is stay calm, when one panics he/she begins to make irrational decisions. If you're able to move do so and try to find out if there is any one else with you in said block, or perhaps in a block next to your own. If at all possible establish a communication system by tapping or knocking on the walls. Obviously you are needed alive or else you would have been either killed or released. So wait for a moment when you can observe your surroundings and go on from there.
Sleep, then wait for my opportuity and or face my consequences... But who knows how I will really react in that situation only one way to find out. Where is the box.