Basically Tony leaves Steve sleeping in bed, after a battle with bad guys the day before. Hours later he discovers that not only is Steve still sleeping, he hasn't even changed position. He can't wake Steve, and the team is called in. Steve isn't just sleeping, he's in a coma from the engery expenditure the day before, possibly being injured?, and going to bed (and possibly burning more calories during sex?) without eating. Once they figure this out, they can treat it and Steve eventually wakes and Tony takes care of him.
*Found!* Link in the first comment!