Sensory overload is a common reaction to a situation in which disbelief supplants the ability to think. It's a sign of unpreparedness. That is, one who encounters a situation in which one is either too frightened to act or unable to do so for reasons of disbelief, should never have gone there. One who flies and airplane and is unprepared to handle an engine failure, fire, loss of directional control, communications loss, etc, shouldn't be flying that airplane.
An instructor who lets a student out on their own who hasn't prepared a student has failed the student. A student who encounters an emergency for the first time on their own has been failed by the instructor. The student should have already been there with the instructor, and the student shouldn't see it as an emergency, but an abnormal situation which has been made routine. Expect the engine to fail. Expect a fire. Expect an electrical failure. Never assume these things are a matter of if, but when. Not if they will happen, but merely a matter of when. Be prepared for the when.
You should never be aloft without having a place picked out, moment by moment, where you'll go in an emergency. A place to divert, a place to make a forced landing, a decision made. We refer to this as being ahead of the airplane. Sometimes pilots fly so far behind their airplane that they'll never be hurt by a crash...they'll arrive 30 minutes later. It's said that one the airplane should never go where your mind hasn't been 30 seconds earlier.
Dealing with the unexpected events which arise is largely a matter of letting go. Don't be tied to what was, several second ago. That's ancient history. Own what is now. Abandon the past, abandon your need for security by asking how this can be happening or why, and simply deal with the airplane in the moment. Work with what you've got. How you've been trained is how you'll fly; train often, review often, and be prepared.
During an emergency is not the time to figure things out...that should have been done before you ever left the ground.