The eye is generally a robust and healthy sense organ, but from time to time serious defects occur either through a natural ageing process of the eye, genetic malfunctioning or a deterioration brought on by other general health issues.
Except in a number of rare cases many disorders do not openly present themselves in a dramatic loss of vision and many do not produce pain. Often disorders occur in one eye only, the other better eye masking the vision loss in the other. This is why many problems go undetected and are usually only discovered by a chance decision to take an eye examination.
As a general rule most eye problems that suffer a loss of vision cannot be reversed but the loss can be arrested, so the message must be to take regular care of your eyes. After all, you only have one pair.