He can't breathe.
There's a pressure deep in his chest and he can't draw in a full breath. That may have something to do with the large chunk of concrete on top of him, he thinks with the increasingly small part of his brain that can think of anything more than the fact that he can't breathe.
He can't breathe and there isn't anybody coming to save him. So he takes as deep a breath as he can and pushes against the concrete on top of him.
It takes a few tries, but eventually he moves the weight enough to be able to wiggle out. He does, and takes several free breaths on the ground before standing.
Spider-Man has a day to save