Usually, this is a very symbolic event in a dream.
A 34-year-old man reports: I dream of being in a situation where I need to act resourcefully to help a stranger avoid danger. Suddenly, I go blind for no apparent reason! It is very frustrating.
Becoming suddenly impaired in this way is different than being injured in a physical accident. The lights just seem to go out without explanation. With a dream like this, it is questionable whether or not the dreamer feels competent to fulfill his duties in waking life. However, this can also refer to his reluctance to accept the challenge of the hero self.
Seeing oneself as a hero is kind of daunting, and the fact that it is your dream doesn't mean that you will necessarily and easily assume that role. Suddenly, the awareness of caring for those to whom you have no obligation is quickened. It's a hassle. Many of us can barely fulfill responsibilities to the people around us in ordinary situations.
Another scenario for loss of a sensory ability is to exchange it for something or someone else. The old saying, I'd give my eye teeth for ... articulates the human willingness to exchange one ability or attribute for something else of value. There are many times when our minds use the principle of exchange to help us verify the relative worth of relationships or objects.
There can also be a distinct martyr image attached to this kind of loss. This is especially true when the dream includes loss of ability through some potentially painful means. The loss may be seen as an exchange for something that was gained during the dream or in waking life.