When you get traped inside an elevator, nobody comes despite the alarm, what do You do? Force the door?, try to escape through the roof? is there enough oxigen inside? what about carbon monoxide?

There is plenty of air, you won't run out of oxygen. If no one answers the emerg alarm then try all the buttons, make sure emerg stop is released. Make sure inside door is closed all the way, open as far as you can then shut all the way. It is hard to pry a door from the inside. Also roof hatch may be locked from the top, but this is another option if nobody answers.