In "The Pit and the Pendulum" by Edgar Allan Poe, why is the narrator afraid to open his eyes when he awakes to find himself on his back with no...

The narrator is afraid because he does not know where he is. 

"The Pit and the Pendulum" is a chilling horror story of a man who is the victim of the Spanish Inquisition.  He has been condemned to death, but he has no idea how he will be killed.  The terror of not knowing what is happening or going to happen consumes the narrator. 

Since his capture, the narrator keeps fainting and waking up.  When he wakes up, he is afraid to look around him.  He does not know what he will find, and he does not want to find out.  If he opens his eyes, everything will become more real. 

There I suffered it to remain for many minutes, while I strove to imagine where and what I could be.  I longed, yet dared not to employ my vision.  I dreaded the first glance at objects around me.  It was not that I feared to look upon things horrible, but that I grew aghast lest there should be nothing to see. 

When he does open his eyes, he does not feel any relief.  The room is so dark that, even with his eyes open, he gets no answers.  The narrator finds this frightening, too:

The blackness of eternal night encompassed me. I struggled for breath. The intensity of the darkness seemed to oppress and stifle me. The atmosphere was intolerably close. I still lay quietly, and made effort to exercise my reason.  

Eventually, the narrator realizes the pendulum is swinging toward him and will eventually cut him in half, so his situation will not really improve.  Sometimes knowing does not help.  Even though the narrator knows there is a pendulum, he does not know when it will finally kill him.  It is a terrible torture, actually.  He has to lay there for days, and he knows not how long, waiting for death.