It could feel like a personal affront that a player doesn't show, especially if you've spent a lot of time in preparation. Your second reaction might be to panic or worry because the absent player controls a critical character or because the missing player is key to your group. The first thing to do is calm down. Deep breaths actually do work because they get the oxygen flowing to your brain again. Next, try to get some perspective on the situation.
Chances are that the offending player really does want to play, but real life just got in the way.
It's nothing personal. So, don't take offense. And, here's an opportunity to stretch your GMing skills and think on your feet. Consider this a challenge, not a problem. Consider all your options, make a decision and move on.