In a completely dark room, if you hold up a mirror in front of you, will you see a
reflection of yourself in the mirror?
An image is formed due to reflection of light by a plane mirror.
In a completely dark room, there is no light present in the room which the mirror can reflect.
Thus, no reflection of light takes place by the mirror placed in the dark room. Hence, no image will get formed by a mirror in a completely dark room.