Check the walls, knock on them and check if they are hollow, if they are hollow its most likely another room or outside. and if you knock on the wall and find the weakest area you might be able to break it.
Another thing is if it rains, the walls will erode and mold which makes it weaker.
But how on earth did you even get in there in the first place? a vent?
Not only is it dark, but there is nothing to stand on to reach the ceiling (which all depends if it's ceiling tiles or not), there is no window to break, or door to slam against. There is no way out. No one can let you out unless again the ceiling.
